Looking at the specs the CC121 is a rather dumb device. It sends MIDI notes on a button press, CCs on encoder rotation and PB values on the fader. The other way works the same, so MIDI notes turn the light on and off and PB moves the fader. Really simple actually. All the rest of the logic is done in software on the host side.
This way the plugin that allows the usage of the CC121 with reaper was able to add several clever things that I’d really like to see on the Cubase CC121 integration as well. As an example it can work with most EQs out there, it can swap the pan encoder and the fader to write pan automation or it can disable the motor fader with a button click so you don’t have to pull the wall wart to shut it up in a silent environment etc.