It is plastic foundation. Although. There's one frame in there with no foundation, and that's the nicest one.
But is it worker bee cells or drone cells? The bees love to make drones on the foundationless frames.
The bees will comb the upper box when they are pressed for space. Meaning more baby bees hatch out, or they are lacking space for nectar. If for some reason they fill up the bottom box and haven’t started to move up, take 2 or 3 frames of brood and move them up and squish the remaining brood frames in the bottom box together and fill the sides with new frames.
Different strokes for different folks I guess, but I always pull the outer most frame first. And then pry and slide the rest as needed for an inspection. The odds of the queen being between that outer frame and the box is very small. There’s almost never brood there unless the bees are really cramped. I might end up killing a few more workers that way, but I feel I’m less likely to squish the queen.