

if the diagram’s any good, it should show the wave stirrer in the roof rather than on the ceiling of the food chamber. There’s typically a waveguide to take the microwaves from the magnetron to the top of the chamber, then the wave stirrer is at the end of the waveguide to vary the angle that microwaves enter the chamber at. There’s usually something to stop food splashing/spraying into that section, though, e.g. an extra few centimetres of waveguide afterwards with a bend in it.
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.