Why do the metal fins in the refrigerator get warm? what is the source of this heat?
A refrigerator works by taking energy out of one area and moving it to another area.
The heat itself comes from the outside. The refrigerator is really just collecting any external energy that would normally just heat up a box in your kitchen to room temperature, and keeping it out of that box.
This is the basic premise of a heat engine and because this is imperfect, the heat gets concentrated in places, such as "the metal fins".
That heat was removed from the objects you put in the refrigerator, or perhaps from warm air that went in there while you held the door open.
When you put a drink that is at room temperature in the refrigerator, heat has to be removed from that drink to cool it. That heat initially warm up the air around the drink. But the wall of the refrigerator's interior is cold, so the warm air gives its heat to the wall.
That heat has to go somewhere, so the mechanism of the refrigerator moves it to the fins so that the air of the room will take that heat and disperse it.
I hope this helps,