Regarding your ? on layers, what you see in the vid are the tabs. They’re sort of like separate sheets of paper in a sketchpad. You can merge or switch the two layers, can copy and paste tabs, and load images as tracing paper. You can also create stencils, as you saw in the vid.
So it’s not meant to work like Photoshop. It’s just a great way to draw and paint. I honestly can’t wait to see what you crazy good artists create. There’s already some good stuff being posted. 
Regarding the stencil, I did beta test, but didn’t have time to really dig in to the features. I just started drawing. So I’ve only tried the brushes and canvas textures.