When you open the door of a refrigerator or an air-conditioned room, the air that comes out will be warmer than the air inside the fridge or room. This is because the inside of the refrigerator or the AC room is cooler than the surrounding environment.
In the case of a refrigerator, it works by removing heat from the inside to keep it cool. When you open the door, the warmer air from the outside rushes in to replace the colder air that has settled inside. This is why you can feel a gust of warm air when you open the fridge door.
Similarly, in an air-conditioned room, the AC system cools the air inside by removing heat. When you open the door, the warmer air from the surrounding area will enter the room to replace the cooled air.
The principle at play here is that heat naturally flows from warmer areas to cooler areas, which is why the warm air from outside moves in when you open the door. The same concept applies to any enclosed space where there is a temperature difference between the inside and outside.