Since it doesn't use any graphics API, what tools do you use and how do you make this game cross platform? Do you use glfw or some audio library or do you write that yourself too?

Hey! To make it cross platform I write backends for the different platforms.

A few platform specific things are needed to be done:

1. Making a window that I can draw an image on
2. Getting keyboard and mouse input
3. Audio

On macOS I use X11 and AudioToolbox, on Linux I use X11 and ALSA, and on Windows I use Win32 + GDI + WinMM

I have some code on GitHub that demonstrates some of these things (minus the audio):

https://github.com/LMP88959/PL3D-KC