OK, this is basically just about any non-default OS API running on all different OS. But for my example let´s consider platform Windows, API SDL (Simple DirectMedia Layer).
Actually this question came to my mind when I was reading about SDL. Originally, I thought that on Windows (and basically any other OS) you must use OS API to make certain actions, like writing to screen, creating window and so on, because that API knows what kernel calls and system subroutines calls it has to do. But when I read about SDL, I surprised me, because, you cannot make computer to do anything more than OS can, since you cannot access HW directly, only thru OS API, from Console allocation to DirectX.
So, my question actually is, how does this not-default-OS APIs work? Do they use (wrap) original system API (like MFC wraps win32 api)? Or, do they actually have direct access to Windows kernel? Or is there any third, way in between?
Indeed, SDL is a wrapper for OS-specific calls, although with many simplifications and convenience functions. On Windows, SDL uses DirectX.