C++ error: Variable does not name a type when attemtping to access global variables

Within a program i am writing i have 3 files. main, functions and a header file. The main file makes use of the header #include "implementation.cpp", and the function file makes use of the header #include "driver.h". Within the implementation file 3 structs have been declared, each sharing their values through the use of extern, too be seen in the minimal reproducing code required. Within the header file i have declared 3 variables to use these extern variables. I believed this was enough to fix my issue but problem still aries. When my code attempts to compile i recieve the following error messages. There are alot but I believe they all stem from the same issue. That issue being the header file able to locate the types Customer, Parts and Builder

In file included from implementation.cpp:8,
                 from driver.cpp:4:
driver.h:7:1: error: 'Customer' does not name a type
    7 | Customer myCustomer;
      | ^~~~~~~~
driver.h:8:1: error: 'Builder' does not name a type
    8 | Builder myBuilder;
      | ^~~~~~~
driver.h:9:1: error: 'Part' does not name a type
    9 | Part myPart;
      | ^~~~
driver.h:14:13: error: 'Part' was not declared in this scope
   14 | std::vector<Part> readpartFile();
      |             ^~~~
driver.h:14:17: error: template argument 1 is invalid
   14 | std::vector<Part> readpartFile();
      |                 ^
driver.h:14:17: error: template argument 2 is invalid
driver.h:16:13: error: 'Customer' was not declared in this scope
   16 | std::vector<Customer> readcustomerFile();
      |             ^~~~~~~~
driver.h:16:21: error: template argument 1 is invalid
   16 | std::vector<Customer> readcustomerFile();
      |                     ^
driver.h:16:21: error: template argument 2 is invalid
driver.h:18:13: error: 'Builder' was not declared in this scope
   18 | std::vector<Builder> readbuilderFile();
      |             ^~~~~~~
driver.h:18:20: error: template argument 1 is invalid
   18 | std::vector<Builder> readbuilderFile();
      |                    ^
driver.h:18:20: error: template argument 2 is invalid
driver.h:20:24: error: 'Customer' does not name a type
   20 | float complexity(const Customer& c, const std::vector<Part>& parts);
      |                        ^~~~~~~~
driver.h:20:55: error: 'Part' was not declared in this scope
   20 | float complexity(const Customer& c, const std::vector<Part>& parts);
      |                                                       ^~~~
driver.h:20:59: error: template argument 1 is invalid
   20 | float complexity(const Customer& c, const std::vector<Part>& parts);
      |                                                           ^
driver.h:20:59: error: template argument 2 is invalid
driver.h:22:40: error: 'Part' was not declared in this scope
   22 | void robotComplexity(const std::vector<Part>& vecB, const std::vector<Customer>& vecC);
      |                                        ^~~~
driver.h:22:44: error: template argument 1 is invalid
   22 | void robotComplexity(const std::vector<Part>& vecB, const std::vector<Customer>& vecC);
      |                                            ^
driver.h:22:44: error: template argument 2 is invalid
driver.h:22:71: error: 'Customer' was not declared in this scope
   22 | void robotComplexity(const std::vector<Part>& vecB, const std::vector<Customer>& vecC);
      |                                                                       ^~~~~~~~
driver.h:22:79: error: template argument 1 is invalid
   22 | void robotComplexity(const std::vector<Part>& vecB, const std::vector<Customer>& vecC);
      |                                                                               ^
driver.h:22:79: error: template argument 2 is invalid
driver.h:24:38: error: 'Customer' was not declared in this scope
   24 | double variability(const std::vector<Customer>& customerList, const std::vector<Builder>& builderList);
      |                                      ^~~~~~~~
driver.h:24:46: error: template argument 1 is invalid
   24 | double variability(const std::vector<Customer>& customerList, const std::vector<Builder>& builderList);
      |                                              ^
driver.h:24:46: error: template argument 2 is invalid
driver.h:24:81: error: 'Builder' was not declared in this scope
   24 | double variability(const std::vector<Customer>& customerList, const std::vector<Builder>& builderList);
      |                                                                                 ^~~~~~~
driver.h:24:88: error: template argument 1 is invalid
   24 | double variability(const std::vector<Customer>& customerList, const std::vector<Builder>& builderList);
      |                                                                                        ^
driver.h:24:88: error: template argument 2 is invalid
driver.h:26:34: error: 'Builder' was not declared in this scope
   26 | std::vector<double> buildAttempt(Builder b, double variaiblity, double complexityRobot);
      |                                  ^~~~~~~
driver.h:26:45: error: expected primary-expression before 'double'
   26 | std::vector<double> buildAttempt(Builder b, double variaiblity, double complexityRobot);
      |                                             ^~~~~~
driver.h:26:65: error: expected primary-expression before 'double'
   26 | std::vector<double> buildAttempt(Builder b, double variaiblity, double complexityRobot);
      |                                                                 ^~~~~~
In file included from driver.cpp:4:
implementation.cpp:43:19: error: ambiguating new declaration of 'std::vector<Part> readpartFile()'
   43 | std::vector<Part> readpartFile() //function to read Builders, Customers and Parts text file
      |                   ^~~~~~~~~~~~
In file included from implementation.cpp:8,
                 from driver.cpp:4:
driver.h:14:19: note: old declaration 'int readpartFile()'
   14 | std::vector<Part> readpartFile();
      |                   ^~~~~~~~~~~~
In file included from driver.cpp:4:
implementation.cpp:77:23: error: ambiguating new declaration of 'std::vector<Customer> readcustomerFile()'
   77 | std::vector<Customer> readcustomerFile()
      |                       ^~~~~~~~~~~~~~~~
In file included from implementation.cpp:8,
                 from driver.cpp:4:
driver.h:16:23: note: old declaration 'int readcustomerFile()'
   16 | std::vector<Customer> readcustomerFile();
      |                       ^~~~~~~~~~~~~~~~
In file included from driver.cpp:4:
implementation.cpp:100:22: error: ambiguating new declaration of 'std::vector<Builder> readbuilderFile()'
  100 | std::vector<Builder> readbuilderFile()
      |                      ^~~~~~~~~~~~~~~
In file included from implementation.cpp:8,
                 from driver.cpp:4:
driver.h:18:22: note: old declaration 'int readbuilderFile()'
   18 | std::vector<Builder> readbuilderFile();
      |                      ^~~~~~~~~~~~~~~
In file included from driver.cpp:4:
implementation.cpp:208:81: error: 'std::vector<double> buildAttempt(Builder, double, double)' redeclared as different kind of entity
  208 | vector<double>buildAttempt(Builder b, double variaiblity, double complexityRobot) {
      |                                                                                 ^
In file included from implementation.cpp:8,
                 from driver.cpp:4:
driver.h:26:21: note: previous declaration 'std::vector<double> buildAttempt'
   26 | std::vector<double> buildAttempt(Builder b, double variaiblity, double complexityRobot);
      |                     ^~~~~~~~~~~~
In file included from driver.cpp:4:
implementation.cpp: In function 'std::vector<double> buildAttempt(Builder, double, double)':
implementation.cpp:230:23: error: no matching function for call to 'complexity(Customer&, int&)'
  230 |  complexity(c,partsVec);
      |                       ^
In file included from implementation.cpp:8,
                 from driver.cpp:4:
driver.h:20:7: note: candidate: 'float complexity(const int&, const int&)'
   20 | float complexity(const Customer& c, const std::vector<Part>& parts);
      |       ^~~~~~~~~~
driver.h:20:34: note:   no known conversion for argument 1 from 'Customer' to 'const int&'
   20 | float complexity(const Customer& c, const std::vector<Part>& parts);
      |                  ~~~~~~~~~~~~~~~~^
In file included from driver.cpp:4:
implementation.cpp:138:7: note: candidate: 'float complexity(const Customer&, const std::vector<Part>&)'
  138 | float complexity(const Customer& c, const std::vector<Part>& parts)
      |       ^~~~~~~~~~
implementation.cpp:138:62: note:   no known conversion for argument 2 from 'int' to 'const std::vector<Part>&'
  138 | float complexity(const Customer& c, const std::vector<Part>& parts)
      |                                     ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
implementation.cpp:243:23: error: no matching function for call to 'complexity(Customer&, int&)'
  243 |  complexity(c,partsVec);
      |                       ^
In file included from implementation.cpp:8,
                 from driver.cpp:4:
driver.h:20:7: note: candidate: 'float complexity(const int&, const int&)'
   20 | float complexity(const Customer& c, const std::vector<Part>& parts);
      |       ^~~~~~~~~~
driver.h:20:34: note:   no known conversion for argument 1 from 'Customer' to 'const int&'
   20 | float complexity(const Customer& c, const std::vector<Part>& parts);
      |                  ~~~~~~~~~~~~~~~~^
In file included from driver.cpp:4:
implementation.cpp:138:7: note: candidate: 'float complexity(const Customer&, const std::vector<Part>&)'
  138 | float complexity(const Customer& c, const std::vector<Part>& parts)
      |       ^~~~~~~~~~
implementation.cpp:138:62: note:   no known conversion for argument 2 from 'int' to 'const std::vector<Part>&'
  138 | float complexity(const Customer& c, const std::vector<Part>& parts)
      |                                     ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
driver.cpp: In function 'int main()':
driver.cpp:24:27: error: no matching function for call to 'complexity(Customer&, int&)'
   24 |     complexity(c, partsVec);
      |                           ^
In file included from implementation.cpp:8,
                 from driver.cpp:4:
driver.h:20:7: note: candidate: 'float complexity(const int&, const int&)'
   20 | float complexity(const Customer& c, const std::vector<Part>& parts);
      |       ^~~~~~~~~~
driver.h:20:34: note:   no known conversion for argument 1 from 'Customer' to 'const int&'
   20 | float complexity(const Customer& c, const std::vector<Part>& parts);
      |                  ~~~~~~~~~~~~~~~~^
In file included from driver.cpp:4:
implementation.cpp:138:7: note: candidate: 'float complexity(const Customer&, const std::vector<Part>&)'
  138 | float complexity(const Customer& c, const std::vector<Part>& parts)
      |       ^~~~~~~~~~
implementation.cpp:138:62: note:   no known conversion for argument 2 from 'int' to 'const std::vector<Part>&'
  138 | float complexity(const Customer& c, const std::vector<Part>& parts)
      |                                     ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
driver.cpp:26:53: error: no matching function for call to 'complexity(Customer&, int&)'
   26 |     writeFile(buildAttempt(b, complexity(c, partsVec), variability(customerVec, builderVec)));
      |                                                     ^
In file included from implementation.cpp:8,
                 from driver.cpp:4:
driver.h:20:7: note: candidate: 'float complexity(const int&, const int&)'
   20 | float complexity(const Customer& c, const std::vector<Part>& parts);
      |       ^~~~~~~~~~
driver.h:20:34: note:   no known conversion for argument 1 from 'Customer' to 'const int&'
   20 | float complexity(const Customer& c, const std::vector<Part>& parts);
      |                  ~~~~~~~~~~~~~~~~^
In file included from driver.cpp:4:
implementation.cpp:138:7: note: candidate: 'float complexity(const Customer&, const std::vector<Part>&)'
  138 | float complexity(const Customer& c, const std::vector<Part>& parts)
      |       ^~~~~~~~~~
implementation.cpp:138:62: note:   no known conversion for argument 2 from 'int' to 'const std::vector<Part>&'
  138 | float complexity(const Customer& c, const std::vector<Part>& parts)
      |                                     ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~

Below is the minimal amount of code needed to reproduce this error. For convience i am only going to include code relevent to Part, as the base code for customer and builder is the same

my main file

#include <iostream>
#include <string>
#include "implementation.cpp"
#include <fstream>
#include <vector> 
#include <random>
using namespace std;

int main()
{
    Part p;
    auto partsVec =  readpartFile();return 0;
}

my header file

Part myPart;
std::vector<Part> readpartFile();

From this point it is noted that the program does not recongise Part myPart, therefore declaring the vector as type Part does not work. Due to this these errors occur.

driver.h:9:1: error: 'Part' does not name a type
    9 | Part myPart;
      | ^~~~
driver.h:14:13: error: 'Part' was not declared in this scope
   14 | std::vector<Part> readpartFile();
      |             ^~~~

Finally here is the implementation file with code relevent to Part

#include <iostream>
#include <string>
#include <sstream> 
#include <fstream>
#include <algorithm> 
#include "driver.h"
#include <random>
#include <vector>
#include <time.h>
using namespace std;

struct Part {
char partCode;
std::string partName;
int maximum;
int minimum;
int complexity;
};
extern Part myPart;

std::ifstream partsList("Parts.txt");


std::vector<Part> readpartFile() //function to read Builders, Customers and Parts text file
{
    std::vector<Part> parts;
    std::string line;

    while (std::getline(partsList, line))
    {
        line.pop_back(); //removing '.' at end of line
        std::string token;
        std::istringstream ss(line);
        Part part;

        std::getline(ss, token, ':');
        part.partCode = token[0];
        std::getline(ss, part.partName, ':');
        std::getline(ss, token, ':');
        part.minimum = std::stoi(token);
        std::getline(ss, token, ':');
        part.maximum = std::stoi(token);
        std::getline(ss, token, ':');
        part.complexity = std::stoi(token);
        parts.push_back(std::move(part));
    }

    return parts;
    }

This code shuould be enough to provide anyone with the means to find out my issue and possible rectify the issue. Thankyou

P.S it should be noted that I have tried putting my structs in my header file, which does not work.