In tail calls, how do programming languages know what the function call evaluates to?

Question

My question title could be improved, if there's a specific name for what I will talk about let me know.

This isn't for a specific language either, all the ones I've used treat function calls as expressions the same.

So I've been reading about recursion and tail calls, so I wrote this code in C++

#include <iostream>
using std::cout;

int test(int num) {
    if (num > 0) {
        return test(num - 1);
    }
}

int fact(int num) {
    return num == 0 ? 1 : num*fact(num - 1);
}

int main() {
    cout << test(20) << '\n';
    return 0;
}

Of course test(num) would always evaluate to 0 if num > 0, since base case is n = 0.

But why? How does the language know what should be returned? How does it know what test(n - 1) should evaluate to?

Edit;

I've included a recursive method of getting the factorial of a number. How would C++ (or any language) know what to multiply num by?