Any guess as to what type of network layer they use there?

I thought cables are short enough in a car that they can attached the displays directly to rendering hardware. Are there other advantages? Provide a fallback in case one render process fails?

Not really, but car manufacturers have actively used network based electronics for easily twenty years or more. (In the early 2000's, I did some work on CAN interfaces for PC's that were often used to connect them to car hardware.)

The network simplifies the wiring and reduces the amount of wire used. Material cost reductions across the life of the car (from design to use).

At one point BMW were very into their fibre networks in-car, but I think they moved away from that for reliability. EDIT: here it is. https://en.m.wikipedia.org/wiki/MOST_Bus