Krita is cross-platform, MyPaint and GIMP are sort of available on Windows and macOS, but the unterlying technology makes it really hard to use them: you will have even more problems with tablets with those applications.
MyPaint has an infinite canvas, Krita and GIMP have images with a certain size.
Muses is a video course on digital painting. It’s old, but it focuses on teaching digital painting, not finding your way around the UI: https://krita.org/en/support-us/shop/
Sai is an old application that does not do color management and has a very weird encrypted file format.
GIMP is an application that’s more meant for working with photos, Krita and MyPaint are applications meant to paint with.