The holes on the sides in the corners are Kensington lock attachment points. Nano size maybe? I think Kensington calls it nano.
The stylus doesn’t have an on/off switch. I believe it goes into a sort of standby mode when it isn’t being moved around and then sort of “wakes up” when it’s moving around. At least, that’s how I understand most to work when they don’t have a dedicated power on/off method.
I’m sure there are other options for Linux, but I’ve mostly used an app called Krita.