When you kill a player, you copy their inventory.
Original video by TapL: https://www.youtube.com/watch?v=LArGlIgFSaQ
Built on the Paper 1.16 API. Tested on Paper 1.16.1 Build 134.
These are mostly being made for fun, and to prove I know what I'm doing. They are not meant for production use, although they would probably work. These might not always be the most efficient, but they're good enough.