Right. i am at work at home and i’ve just taped in the corners. These are called in the trade internals. As you can see on the left photo that is a taped in internal. you put the plaster on both sides then you put over that the paper tape which you fold(it has a fold line in the centre of it). Once you have that in you blade it off with a 3 inch (angled) broad knife so you don’t spread the plaster to wide. And that’s it you have a taped in 90 degree corner which later on you top coat with a 4 inch blade and a special corner tool. i will explain that later.
cheers
phil