My car (2014) has seven or so different CAN busses, with a gateway in between. Whether that constitutes a different "network" or not can be argued, but at least something that brings the entertainment CAN bus down, be it just by e.g. shorting it, won't (easily) bring down the other busses.
No security or authentication. IIRC, any attached device has full control over the physical layer by design. I don't really see why this is a problem in a car with isolated buses for safety-critical components.
Many newer vehicles contain systems, OEM and/or after-market, that are permanently connected to the internet via cellular modem. Other systems with insecure RF tech used for various gimmicks. Other systems that communicate with external and potentially malicious devices like chargers. Etc. Most of these systems have enough access to (in)directly destroy or booby trap the car. My car is able to receive ECU(!), firmware, software updates OTA from the manufacturer. These critical systems are just as "closed" or "isolated" as cloud-enabled "CCTV." Scary stuff.
Somewhat unrelated to this particular case, but terrifyingly, the network isn't and CAN is one of the least secure BUS's imaginable.