Operators overloading in c++

I recently found out about overloading operators in c++. When you want to overload a operator in class , and we want to create new object with it , that is made from others we defined we can do

NameOfClass operator+(const NameOfClass& b){
  {
         NameOfClass tmp;
         tmp.length = this->length + b.length;
         tmp.breadth = this->breadth + b.breadth;
         tmp.height = this->height + b.height;
         return  tmp;

}

I cant figure out if i define 2 objects before this. e.g

NameOfClass one(length,breadth,height);
NameOfClass two(length,breadth,height);

I set their attributes. but how does

NameOfClass three=one+two;

set the attributes of "three"? Are both one and two considered as argument to "+" overloaded operator. The function clearly says

tmp.length = this->length + b.length;

but this-> length should be undefined and b.length is private. How does it mix it together? Using example from tutorialspoint.

#include <iostream>
using namespace std;

class Box
{
   public:

      double getVolume(void)
      {
         return length * breadth * height;
      }
      void setLength( double len )
      {
          length = len;
      }

      void setBreadth( double bre )
      {
          breadth = bre;
      }

      void setHeight( double hei )
      {
          height = hei;
      }
      // Overload + operator to add two Box objects.
      Box operator+(const Box& b)
      {
         Box box;
         box.length = this->length + b.length;
         box.breadth = this->breadth + b.breadth;
         box.height = this->height + b.height;
         return box;
      }
   private:
      double length;      // Length of a box
      double breadth;     // Breadth of a box
      double height;      // Height of a box
};
// Main function for the program
int main( )
{
   Box Box1;                // Declare Box1 of type Box
   Box Box2;                // Declare Box2 of type Box
   Box Box3;                // Declare Box3 of type Box
   double volume = 0.0;     // Store the volume of a box here

   // box 1 specification
   Box1.setLength(6.0); 
   Box1.setBreadth(7.0); 
   Box1.setHeight(5.0);

   // box 2 specification
   Box2.setLength(12.0); 
   Box2.setBreadth(13.0); 
   Box2.setHeight(10.0);

   // volume of box 1
   volume = Box1.getVolume();
   cout << "Volume of Box1 : " << volume <<endl;

   // volume of box 2
   volume = Box2.getVolume();
   cout << "Volume of Box2 : " << volume <<endl;

   // Add two object as follows:
   Box3 = Box1 + Box2;

   // volume of box 3
   volume = Box3.getVolume();
   cout << "Volume of Box3 : " << volume <<endl;

   return 0;
}