[tfvlrue]

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.

For a visual aid, these are pictures of the replacements parts: https://www.partstown.com/panasonic/PANA010T8K10AP https://www.partstown.com/panasonic/PANF202K3700BP

[ale42]

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.

[IIsi50MHz]

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.