All cars have split CAN buses
CAN packets are prioritised, meaning it is possible to completely DOS a bus through sending high priority messages - that will never allow low priority messages to get through. There is no "fairness" policy etc.
You certainly separate the engine control stuff from the body electronics. Any bridging between these is done with a gateway of sorts (think firewall). This limits what data can flow between the buses as well as the rate packets can be sent.
Even when you attach diagnostics (OBD2 etc) that will be via the bridge preventing the diagnostics tools from trashing the engine bus.
It is also very common for the different buses to have both different bit rates and signalling levels. There is no point in having high speed CAN buses (1M etc) for opening doors, switching on lights etc etc. There is good reason to have high speed in the engine to reduce latency.
So where does a Google device sit on the bus(es)? Clearly not on the engine bus. Most vendors would only allow it to be attached to a dedicated bus so that the bridging can be managed into the rest of the vehicle.
Nothing new to this - that's how CAN buses have been set up "forever".