(however, I don’t get why more loops and ifs makes a function harder to test, I’m just going to trust you and that I’ll find out later.
Well, it's fairly easy to explain - each branching statement in your function doubles the number of discrete paths through the code. If there's one if statement, there's two paths through the code. (The one where the if predicate is True, and the one where it isn't.) If there's two if statements, there's four paths through the code. If there's three if statements, there's eight paths through the code.
In order to test a function completely, you have to test every possible path through the code. If you used three if statements, that means you have to devise and write eight tests just for the different code paths, plus testing various exceptional cases of the function's input ("what if all inputs are 0", "what if all inputs are null", "what if the integer is a string", etc.) That's a lot of tests! You might even have to write tests for exceptional cases combined with different code paths, so now you're writing eight times the number of tests you otherwise would have had to.
Whereas if your function doesn't branch at all, there's only one path through the code to have to test. That's a lot fewer tests which means you'll probably actually write them instead of saying "well, it looks like it works, I won't spend the time on tests right now." Which is how bugs make it all the way through to the end of the project.