Hard to tell for sure from the photo but it looks like an old floor register. If there is no air flow coming from it then my guess is that the previous owners at some point renovated and moved some walls around and rerouted the supply line elsewhere and instead of removing the old line just decided to leave it since it was inside of the wall. As for return air, usually it is done with one large duct but sometimes it is done with multiple if there is more than one floor or the furnace/heat pump supplies multiple units in a house but it is not usually in every room. As long as the air can transfer out of the rooms (ie. most doors have a gap underneath them) and make it to the return air duct that’s fine.