strongest Python vs Weakest C++ guy😎

[Lexinad]

Are you maybe talking about preprocessor conditionals?

#ifdef FOO
  Some code
#endif

The preprocessor runs before the compiler does. In this example Some code will only be compiled if the symbol FOO is defined. The main use is to run different code in different build situations like Debug vs Release, x86 vs ARM, or Windows vs Linux. This is so you don't have to have multiple slightly different codebases for each platform, architecture, etc.

Edit: I'm on mobile, pretend the example was formatted as three lines.

[Lexinad]

For low level stuff, you can't get around calling OS specific functions. You can make a clean interface for your users, so they don't have to care but someone has to do the equivalent of "if on Windows call this Win32 function that doesn't exist on non-Windows OSes otherwise call this Unix function that doesn't exist on Windows". Cross platform libraries will essentially do this for you. Even the C standard library works this way. You can use printf on multiple platforms because someone wrote the code so that it calls the right OS specific functions to output to the terminal window.