I'm learning c and i'm at references at the moment and i'm trying to be thorough, in this article (https://www.geeksforgeeks.org/references-in-cpp/) in the second code block is the following example

// C   Program to demonstrate
// Passing of references as parameters
#include <iostream>
using namespace std;

// Function having parameters as
// references
void swap(int& first, int& second)
{
    int temp = first;
    first = second;
    second = temp;
}

// Driver function
int main()
{
    // Variables declared
    int a = 2, b = 3;

    // function called
    swap(a, b);

    // changes can be seen
    // printing both variables
    cout << a << " " << b;
    return 0;
}

My first thought is why can you pass a normal variable to a function that takes a reference as a parameter?

https://www.w3schools.com/cpp/cpp_references.asp :

string food = "Pizza";  // food variable
string &meal = food;    // reference to food

Shouldn't it be a reference variable like meal from above?

I decided to make a small test to see with both a string and a struct:

void Lesson83a(string varIn, string newValue) {
  varIn = newValue;
}
void Lesson83b(string& varIn, string newValue) {
  varIn = newValue;
}

struct Lesson83Struct {
  string testString;
  int testInt;
};

void Lesson83c(Lesson83Struct structIn, string newS, int newI) {
  structIn.testString = newS;
  structIn.testInt = newI;
}
void Lesson83d(Lesson83Struct& structIn, string newS, int newI) {
  structIn.testString = newS;
  structIn.testInt = newI;
}

void Lesson83() {
  string testString = "Foo";
  string& testStringRef = testString;
  Lesson83a(testString, "1");
  cout << testString << "\n";
  Lesson83b(testString, "2");
  cout << testString << "\n";
  Lesson83a(testStringRef, "3");
  cout << testString << "\n";
  Lesson83b(testStringRef, "4");
  cout << testString << "\n";

  cout << "-----\n";

  Lesson83Struct testStruct;
  Lesson83Struct& testStructRef = testStruct;
  testStruct.testString = "Foo";
  testStruct.testInt = 0;
  Lesson83c(testStruct, "1", 1);
  cout << testStruct.testString << " " << testStruct.testInt << "\n";
  Lesson83d(testStruct, "2", 2);
  cout << testStruct.testString << " " << testStruct.testInt << "\n";
  Lesson83c(testStructRef, "3", 3);
  cout << testStruct.testString << " " << testStruct.testInt << "\n";
  Lesson83d(testStructRef, "4", 4);
  cout << testStruct.testString << " " << testStruct.testInt << "\n";

}

Foo
2
2
4
-----
Foo 0
2 2
2 2
4 4

1. It makes no difference whether you give a normal variable or a reference to a function that expects a normal one

2. a function that expects a reference can modify the variable send to it outside its scope regardless of it you pass it the variable directly or a reference to it

The first article states that one of the reasons to use references is to "Avoiding a copy of large structures" so this seems important to me, my question is what happens under the hood in the compiler? does it matter in which way you do it?