Properly tuned and stocked fridges.

Fridges work by pumping a lot of heat out of an air stream that then goes into the freezing compartment. From there, it pumps air into the fridge as needed through a vent on the side or back.

There’s also a thermostat in the fridge, usually as far away from the air inflow as possible. It determines how much cold air goes into the fridge before the fan turns off, and how warm the fridge compartment can get before it turns on again.

If you block the air inflow, whatever you block it with is essentially in the freezer, and since all the energy is going into keeping it frozen, the fan stays on a long time before the thermostat gets cold enough to turn it off.

Likewise, if you block the thermostat with something warm, the entire fridge will freeze before the fan turns off.

Conversely, if you put something in the fridge to defrost and place it against the thermostat, that fan will stay off and the fridge will warm up and stuff may spoil. If you put something to defrost against the inflow, it’ll stay frozen for a long LONG time.

The more full the freezer is, the more it will keep itself cool, and the colder the air will be flowing into the fridge inflow vent.

This is why sometimes you need to adjust the climate control dials in the fridge from the default settings.