C++ overloading the << operator

I am trying to overload the << operator in c++, I have implemented the code properly but I have a few doubts about the overall working of the overloading concept.

1. Why does the ostream object in operator<< function need to be passed by reference?

2. Why cant the return type of the function be 'std::ostream' instead of 'std::ostream&' ?

3. Is it the property of the ostream class that requires the 'operator<<' function to return a memory address or do all the operator overloading need to be passed by reference and the return type should also be a reference?

Here is the code.

    #include<iostream>
    enum MealType{NO_PREF,REGULAR,LOW_FAT,VEGETARIAN};

    struct Passenger{
            char name[30];
            MealType mealPref;
            bool freqFlyer;
            int passengerNo;
    };

    //Overloading the << operator

    std::ostream& operator<<(std::ostream& out,const Passenger& pass){
            out<<pass.name<<" "<<pass.mealPref;
            if(pass.freqFlyer){
                    out<<" "<<"Passenger is a freqent flyer ";
            }
            else{
                    out<<" "<<"Passenger is not a frequent flyer ";
            }
            return out;
    }

int main(){
        Passenger p1 = {"Jacob",LOW_FAT,false,2342};
        std::cout<<p1;
        return 0;
}

EDIT 1: Why do i have to use the << operator in the overloading function definition. Does it function like a bitwise shift or like << in std::cout<<" ";