I recently got a Wacom Bamboo Fineline 3, a stylus that is ment to be used on iPad and iPhone with compatible apps and just generally a thing that can interact with the screen, can I use this in Stylus-only mode, or even at all? Or do I have to use the Apple Pencil or my fingers?
Probably. When I picked up my Apple Pencil on first use of PS for Mobile the pencil battery was discharged, so I grabbed a cheap rubber-tipped stylus instead. Worked well enough. Your stylus will be better.