To add a layer, press the + button.
I don’t really know how to explain it well, so I’ll try my best!
When you add a layer, you can rearrange them to your liking by holding down on the square part that shows the canvas and dragging it… by having a layer under one of the layers, you can color under it without coloring over it… if that makes sense.
You can use alpha lock to color that without messing it up or coloring out of it.
If you have other questions feel free to ask! I tried to explain it as simple as possible. 