iWhat DougS said.
CANBus's two speeds were traditionally Drivetrain and Infotainment/VAC. The Drivetrain ran at a higher frequency, and the Infotainment at a lower frequency. It runs on a twisted pair of wires, with ground being through the power supply to each module.
It's a packet-based system, with priority. All modules (NXP, Bosh, Whoever) can send and receive, and be either sensors and/or actuators. The high speed version will only run if both wires in the twisted pair are good, the low speed version is tolerant of a fault in either wire.
If you break down, you can still listen to the radio and wind the windows down whilst waiting for the recovery vehicle. So far, so good. Very good, in fact.
Further commands to remote control the vehicle could then be received via the car's built in cellular connection.
Very good, as long as you don't fit a digital wireless receiver to the vehicle's physical network.
I can't think of any reason why a car stereo needs to communicate to the drivetrain. But:
It's not just a bloody stereo these days; it's used to control drivetrain features, such as Sport / Eco modes...
(Not my old van, the £50 Lidl Stereo that plays SD Cards and USB sticks is still working and van doesn't have any built in Sat Nav or cellular radio. Actually TBH, recently it sounds like the capacitors in my stereo are on their way out, but must have got 5 years out of it.)