.h is called a header file, and is basically a library of code that either you make (or downloaded), or it comes with the compiler. Content-wise, it looks just like a C++ source file without a main()

A demonstration will be useful. Do the following:

• Make a C++ file like how you usually do on your computer. Paste this code in.

#include <iostream>
#include "algo/sum.h"
using namespace std;

int main() {
	cout << sum(1, 2) << endl;
}

• Make a folder named algo, in the same folder with your C++ file.
• Make a file named sum.h inside algo, then paste this code.

int sum(int a, int b) {
	return a+b;
}

• Now run the C++ code (the "something.cpp" file)

You can do include in the header file, or add more stuff inside that header file, or add even more header files to the algo folder, or do any customizations you want to make your own algo library.

debug.h is probably just their own custom-made header file for debugging CP codes if needed. You might be able to find some online, or you can make one by yourself.

Stop trying to figure out how um_nik's mind works, you are doing it wrong, go and solve some problems.

Some jokes. IMO, as u accumulating more knowledge and implementing more and more code, at any unknown moments, u could unexpectedly understand some things that could seemed weird before. U would only find something brainy(helpful) when its meeting your current demands/situation.

More specifically, u may understand the use of the mentioned code when u are writing some “large” problems or u simply want to code faster.