[2023] Explain POI's to me please?

The 'end' of a POI is generally demarcated by a loot stash; A typically locked chest of some sort and a number of closed wooden crates and maybe a pile or three of medicine or ammo all in one small area.. The 'end' also tends to have a quick way back out of the building somewhere nearby to it.

But aside from being on a quest from the trader, there's nothing you explicitly have to do to 'finish' a POI; You can just do however much you feel like, leave whenever you want and come back- Zombies'll respawn after a number of days though, so make sure you come back before then if you wanna finish looting.