well, "cognotive" load makes me laugh. you're basically arguing that this is simple:

// main.cpp
#include "math_session.h"
int main(){
    run_math_session(); 
    return 0; 
}


// math_session.h
#pragma once
void run_math_session();


// math_session.cpp
#include "math_session.h"
#include <iostream>
using namespace std; 

void run_math_session(){
    int a, b; 
    cout << "input number a: " << endl; 
    cin >> a; 
    cout << "input number b: " << endl; 
    cin >> b; 
    cout << "the sum is: " << (a+b) << endl; 
}

but this is hard:

// main.cpp
#include <iostream>
using namespace std; 

int main(){
    int a, b; 
    cout << "input number a: " << endl; 
    cin >> a; 
    cout << "input number b: " << endl; 
    cin >> b; 
    cout << "the sum is: " << (a+b) << endl; 
    return 0; 
}

and tbh, this view just makes me think "ok, you do your thing, i do my thing"