Thanks for your response (I have used software like Remote Desktop). If that is what we are going to get in our Teslas, I like it. And I think it makes sense for Tesla.
The way CarPlay and Android Auto work is that your phone creates the actual display content, which is relayed to the car for display. Essentially, you can think of them as using the car as an external monitor. It's a bit more complex than that as the car gets a custom output different than what's displayed on the phone itself, and it can convey control information like taps on the touchscreen back to your phone for processing, but that's the general idea. They would have access to, at a minimum, the car's display (or a window within the display), the speakers, and the car's microphone.
If you've used Remote Desktop or VNC to use a remote computer before, you have the general idea of what's involved. It's essentially that with a fancy interface on top.