They are basically a mini fridge with thermoelectric cooling unit. The part used to cool is called a peltier device, which is basically an electronic device with a hot side and cold side. They install heatsink and cooling fan on hot side, the cooler it is the colder the cold side will be. Then use temp controller to cycle electronics on and off via setpoints. They don’t cool as well as standard refrigeration unit, but they also don’t play havoc on relative humidity within the space.
If you are handy and resourceful, you can build one out of old mini fridge uber cheap. I think I got 5- 50 watt peltier devices for around $10 on aliexpress. Spent $18 on power supply, then used a bunch of stuff I had leftover from lighting projects to complete. I never bought controller, but something that small should be available on Amazon for +/- $20. Could probably use same stuff everyone is using for diy rosin press.