I’ve never used or dissassembled an American microwave, so wouldn’t know about anything American-specific.

Having a ceramic plate at the end of the wave guide isn’t evidence that there’s not a wave stirrer on the other side of the plate or part way along the wave guide. As far as the microwaves are concerned, the ceramic plate is a hole, though, because (unless the manufacturer selected a stupid material), it should be about as transparent to microwaves as the air is.