SIM
It will have an eSIM not a physical SIM, possibly multiple eSIMs, so someone is telling porkies about removing it. However, it will have to connect via an APN and that can be blocked. OTA software updates are required or things get very expensive if you need to physically connect and flash all of the ECUs, and there could be 5-10, maybe more. This will work on a small number of buses but not a large fleet and the software has to come from the manufacturer. This is the same principle as updating your smartphone although safety critical systems are not typically updated OTA.