function of <window.h>?

Answer 1

It's <windows.h> btw. As the header title would suggest, it offers a wide variety of functions for Windows OS. Be warned though, it is huge and resource-heavy.

Answer 2

you can create native windows applications this way. but it us really low level, the code looks cryptical and you have to writr a lot of code to get thr simples things done. if you want to do gui applications with c++ i recommend using a franework like qt. but the winapi can be interesting for understanding what is going down under the hood. basically every gui franeworl used in windows just encapsulates winapi calls.