It's probably spring clips of some variety.

Something like this: https://www.amazon.com/Recessed-Lighting-Accessories-Stainless-Bathroom/dp/B0B6VWYS9R/

Basically the spring arms get put through a little slot and they want to spread out thanks to the spring. As you push the face of the light against the ceiling the springs do spread out and hold it snug. As you pull it down those little loops on the end will catch on the slot but you should have enough clearance that you can reach in and squeeze the arms together and fit everything through the slot to remove the light plate entirely and gain access to the bulb.

Try rocking it back and forth to feel if there's a pivot point. If there is it's probably where those spring clips are, there's probably only 2. If it is spring clips you should be able to just pull down with a couple of pounds of force and that'll be that.

It could also be some sort of twist lock, but probably not with a square fixture.