Havent heard any details. That is a long way away.
As for the access the computers, it would use the same method as they do now. I am not sure what the over the air updates uses for example. But the computers are connected to the Tesla network.
Would Tesla pay the vehicle owners for use of the computers and the battery it uses?
I would think it would be more the applications that are using the computers for inference. If you have "Inference-as-a-Service", there would be different applications that could use the infrastructure. They would have to pay for the service.