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