Proper way to mount TV on wall that might be lath-and-plaster?
The wood that's visible around the edge of that opening is plywood. I would suggest opening up several other boxes throughout the apartment (or at least along that wall) to see if they're all like that.
If this wall seems to have a consistent plywood backing, you're in luck because all it takes is a few of the appropriate sized lag bolts (specified in the instructions of your wall mount) and you're good to go. You can hang that thing anywhere you want - plywood has a lot of strength and can take the weight of just about any TV/mount combo out there. So much so that the frequent recommendation is to attach plywood to the studs then attach the mount to the plywood. You just get to skip the first step.