https://www.wch-ic.com/downloads/CH9329DS1_PDF.html

They dont have the. The chip convert the ecltrical signal into keypress

The ch9329 they link is the chip they use. It is certainliy off the shelf and purchased from another, that does not provide much more information to them past implementation

The firm: Check the https://github.com/sipeed/NanoKVM/tree/main/server there is quite a bit of .so committed to the repo sadly. The NanoKVM in NanoKVM USB use CH9329.

Just isolate it on the network so it doesn't have outside internet access.

There are also similar alternative devices like JetKVM, TinyPilot, etc.