These commercial microwaves have a ceramic tray (transparent to microwaves) that the food sits on. It fills the entire bottom of the microwave. Underneath the tray is a small piece of metal bent into a particular shape attached to a spindle that rotates. The idea being that it spins around the reflected microwaves rather than the food itself.
Same principle in the combined oven/microwave from Siemens we have at home, except the microwaves come from the top. If you look well, you can even see the thing rotating.
Combination microwave + oven (with a single compartment) are win-win. In Japan, they're common and many people wonder why anybody would want a separate microwave and toaster oven: combining them takes less space and takes less time for a better result.
Most microwaves only have the magnetron (the part actually producing the microwaves) on one side. The rotation is needed to cook your food evenly.
This is why food in the middle of the tray often ends up undercooked. No matter how the tray rotates, that part is never particularly close to it.