Arne's painting and pixel art tutorials are really great resources
These two artists post pixel art tutorials to Patreon, useful for other art styles too:
AzPainter and AzDrawing are light-weight art programs for Linux.
MyPaint brushes have been added to OpenToonz, but I've had a problem getting them to work.
The Bezier Game is a way to learn to use the pen tool for Illustrator/Inkscape.