i want to try and use the "/requiem possession" command but dont know how as im kinda confused about how you force yourself to possess a mob forcefully

i think i should clarify, mainly how you know what to type in for the entity you need to control, so if there's some sort of list someone can send me i'd like that

That's a traditional entity selector (@e), you can find information about it on the minecraft wiki: https://minecraft.fandom.com/wiki/Commands#Target_selectors