COMPRESSOR SPEED
A few decades ago, a thermostat would stop the compressor when the refrigerator's temperature had become sufficiently cold. You'd hear the refrigerator fall silent, and then a few minutes later, it would start up again with a loud whir. The thermostat is a mechanical device that relies on the difference in temperature expansion between two metal plates. The two plates are fixed to each other.
A very precise temperature could not be expected with this technique. The refrigerator needed to be insulated with thick walls, and defrosting had to be mandatory.
Today, electric motors have made enormous progress. Coupled with super-fast temperature sensors and powerful processors, they can instantly change their rotational speed.
There The temperature stabilizes, thick walls are no longer needed, and the motor is silent! Hooray for progress!
Unfortunately ! There's a price to pay; this type of refrigerator cannot be used as is on Shabbat!