Microwave ovens work by exciting water molecules using many hundreds of watts of ~2.45 GHz microwaves.
This specific frequency has a heating effect on water, so when you blast enough of it at food, which is often very saturated with water, it will heat up. The heat energy will transfer to the rest of the molecules in the food by contact.
That’s the general idea at least… I’m sure there’s more interactions that happen, water is just the most significant, to my knowledge.
So the protection in the microwave is capable of reflecting (for the purposes of containment) 2.4Ghz microwaves very well, and bluntly, does a good job with many other radio waves too, across a pretty broad band of frequencies… so the material that makes up the protective chassis of a microwave is ideal for making a reflector for wifi, since it was constructed with the idea of reflecting 2.4Ghz frequencies. Microwave ovens create the signal fairly crudely with a magnetron, but the underlying concepts are the same.