Windows Runtime (WinRT) is the "successor"* of Windows API (Win32 API). It supports development in C++/WinRT (standard C++) and C++/CX (Component Extensions, a language based on C++) [C++/WinRT is recommended], as well as other scripting and managed languages.
A good starting point...
https://docs.microsoft.com/en-us/windows/uwp/cpp-and-winrt-apis/
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* I believe Microsoft describe it that way, I'm not sure if it is but it is worth looking at.
EditWindows Runtime (WinRT) is NOT to be confused with Windows RT.
Alternatively there are several 'high level' and somtimes platform-independent GUI-libraries available, as i.e. wxWidgets (that i'm using), Qt, fltk etc..
I use wxWidgets because it is open source, well documented and easy to learn. Also there is not so much 'overhed' needed to be programmed 'manually'. So i.e. a resizable and closeable main-window can be created with only a few program-lines like this: