it's pretty cool once you get used to it, I can tell you what all the tools do and tell you some tips if you want!

the marker tool (the yellow one) is used to color in the lines without coloring on top of them, it lets you color under stuff that's a darker color than the color you're using.

The eraser tool is used to well, erase things.

the pencil tool gives you smaller and sharper lines than the pen tool

the pen tool gives you bold and clean lines, though if you streak it quickly across the page, it'll give you curves

the roller tool, which you seem to know how to use, lets you create shading and lighting on a drawing, even texture if you use the roller with a small point

the other marker tool (the orange and grey one) lets you color just on pre-drawn things, meaning you can't draw on the default grid, but you can color just on the drawn thing. It's good for animation

the dropper tool lets you select a color you already used on the drawing by tapping the desired color in the drawing

and finally, the paint bucket tool lets you fill in areas of the drawing if the lineart is larger than the smallest point and there are no gaps in the lines

np!