Most commercial refrigerators have thermostats inside them in the form of a knob that can be turned for temperature control. When the temperature is set, the thermostat maintains the preferred coldness by managing the flow of electricity to the compressor.
Refrigeration cooling is not achieved by adding cool air, but by eliminating warm air. The thermostat senses when it is too hot or cold. The flow of electricity stops when the air inside of the refrigerator is at the preferred temperature. When the air is too warm, the flow of electricity starts back up, starting the compressor again to adjust the temperature.