Without a rotating plate there will be a hidden metal fan-like object ether in the ceiling or the floor. this is a rotating reflector designed to scatter the waves a bit.
Don't consider this a recommendation(it may well be utter garbage) but my next microwave is going to be a Sharp R-21LCFS mainly because it hits all my key points. no rotating platter, no modes or features, just one big dial to set the timer... it's... well perfect. Plus I was able to find the service manual for it. I always feel better about a product if it has a service manual.
https://www.sharpusa.com/ForBusiness/CommercialCooking/Comme...
"Perfection is reached, not when there is no longer anything to add, but when there is no longer anything to take away."
-- Antoine de Saint-Exupery
