Microwaves aren’t radiation, they’re radio waves, so microwaved foods never become radioactive in the first place.
The way microwaves work is that they have a strong effect on water, twisting its molecules back and forth rapidly. As the water molecules rub back and forth against other molecules, they heat up from the friction.
Luckily nearly all foods have at least a little moisture in them, otherwise they wouldn’t heat up. Even bread has a little water in it, so do cookies and pastry.