My process of learning photoshop on my own was I'd do a piece of art, and whenever I needed help, I'd look up the keywords on youtube. It's how I still learn photoshop and clip and blender and...all the stuff I use. Because, if you do those courses, they tend to only tell you how to do like...photo editing or page layout--you'll waste your time learning a lot of stuff you aren't actually going to use very much.
As an illustrator, you use different tools, so I'd look up "How do I set up a drawing in (whatever your program is)" watch a few videos on that, and just get started drawing. When you run into a troubleshooting issue (like how do I make brushes? or What is a clipping layer?)--look it up with google.
Another helpful thing is to watch artists do paintings that they've recorded. So, if you follow a popular artist who uses your same drawing program, chances are they've recorded themselves and talked about their process either on twitch or on youtube. If you can catch them painting live on twitch, you can even ask them "hey, what tool are you using?" And they'll be happy to talk about their process.
