The microwave radio frequency has been selected so the energy will be strongly absorbed by the water molecules contained within the food or drink in the target area.
This frequency does not happen to be absorbed by cooking oils very much.
An approximate rule of thumb is that a nominally powered microwave oven will penetrate an inch to maybe two inches into ordinary food before all the energy has been absorbed so that's about as far as it will heat "toward the inside" rather than from the inside. Unless it's little things like small potatoes where the needed heat forms through-and-through. For much bigger stuff where the deep part can not be energized directly, it can be good to take it easy so the outer target layer can be kept from overheating long enough for the outer layer itself to cook the inside thoroughly.
Or take a break a couple times to stir a large bowl of soup between heating sessions.
This frequency does not happen to be absorbed by cooking oils very much.
An approximate rule of thumb is that a nominally powered microwave oven will penetrate an inch to maybe two inches into ordinary food before all the energy has been absorbed so that's about as far as it will heat "toward the inside" rather than from the inside. Unless it's little things like small potatoes where the needed heat forms through-and-through. For much bigger stuff where the deep part can not be energized directly, it can be good to take it easy so the outer target layer can be kept from overheating long enough for the outer layer itself to cook the inside thoroughly.
Or take a break a couple times to stir a large bowl of soup between heating sessions.