Do rear licence plate screws have retained nuts behind?
 

One of my rear licence plate screws is loose and feels like nothing behind
Is the best way to get behind it from the trunk or from under bumper? What nut/clip should be there?

There are two barely attached square captive nuts. Breathing on them will frequently cause them to fall off. I recommend using those plastic toggle anchors from home depot/lowes ( ) and then running a sheet metal screw with a washer to hold on the plate on the car.

Seeing the rust on screws, I got four ( 2 for each side) at Home Depot that have big stainless heads.

Size is metric M6 x12mm which is slightly shorter, since too long anyway.

I put 2 on front and back plates with antiseize.

It is a pity they are so poorly secured, i used one of these wall fasteners to sort it out

Yeah, I'd resort to that.