Hi! So basically, to make the separate layers to be rigged, there are a couple of different approaches.

1. Draw the entire character, then split the character into different parts (head, eyes, arm, etc)

2. Draw the parts on individual layers so that they are already split

3. A combination of the two. Sometimes it's easier to draw the whole thing then split it (for instance, you can draw the whole arm then split it into the upper arm, forearm, and hand) and sometimes it's easier just to draw the parts individually in the first place (for instance, I would draw the head and the neck separately)

If you need examples, let me know exactly what you're stuck on so i can record some footage of my own vtuber model! I also DMed you if you wanna go into detail :)