A microwave oven uses microwaves to heat food.
Microwaves are radio waves.
When they are absorbed they are converted directly into atomic motion - heat. Metal reflects microwaves, which is why metal pans do not work well in a microwave oven. In microwave cooking, the radio waves penetrate the food and excite water and fat molecules pretty much evenly throughout the food. There is heat everywhere all at once because the molecules are all excited together . There are limits of course.