How does a refrigerator make things cold?
One-line takeaway
BLUF: It does not make cold. It moves heat out of the cabinet and dumps it into the room, using a refrigerant that evaporates inside the fridge, absorbing heat, then is compressed and condensed outside, releasing it.
Nearly all mechanical cooling, from a kitchen fridge to a data center chiller, is this one cycle at different scales.
Four parts in a loop
The vapor-compression cycle has four parts in a loop. In the evaporator, coils inside the cold compartment, low-pressure liquid refrigerant boils at a temperature below the cabinet's, absorbing heat from the air and food to do it. The compressor then pumps that vapor to high pressure, which also raises its temperature well above room temperature. In the condenser, the coils on the back or underneath, the hot vapor gives up heat to the room and turns back into liquid. Finally an expansion device, often a narrow capillary tube, drops the pressure abruptly, and the liquid becomes cold again and returns to the evaporator. A thermostat cycles the compressor to hold the setpoint.
You cannot make cold, only move heat
The deeper principle is that heat moves on its own only from hot to cold, so moving it the other way costs work, and that work is what the compressor supplies. The trick is phase change: boiling a liquid absorbs a large amount of heat at constant temperature, far more than simply warming a gas would, so a small amount of refrigerant carries a lot of heat per cycle. Changing pressure shifts the boiling point, which lets the same substance boil at freezer temperatures on one side of the loop and condense at room temperature on the other. Because the machine only relocates heat rather than creating cold, a refrigerator running with its door open warms the kitchen: everything it removes, plus the compressor's own energy, ends up in the room.
The cycle stayed, the chemicals changed
The cycle itself has changed little; the refrigerant has changed repeatedly. Early machines used ammonia and sulfur dioxide, which were toxic. CFCs replaced them, then were phased out under the Montreal Protocol for destroying stratospheric ozone. HFCs followed and are now being reduced in turn because they are potent greenhouse gases, with many new domestic fridges using isobutane. The same cycle run in reverse is a heat pump, which is why heat pumps heat buildings so efficiently: they move existing heat rather than generating it. Variable-speed compressors, better insulation and improved door seals have cut household refrigerator energy use dramatically over recent decades, even as the cabinets have grown larger.
The fridge myths that persist
Common misconceptions start with the name. A fridge does not produce cold, because cold is the absence of heat, not a substance to be made. Leaving the door open does not cool a room; it warms it. Setting the thermostat to its lowest number does not chill food faster, since the compressor runs at one rate and only runs for longer. Putting hot food straight in does not damage the appliance, though it does raise the compartment temperature for a while and food safety guidance favors doing it promptly rather than cooling on the counter. And the coils are not decorative: blocked or dusty condenser coils cannot shed heat, so the compressor runs longer and the fridge uses more power.