Theoretically if the systems are properly decoupled and airgapped than the hacking surface area should be quite low. You might be able to hijack the music coming through the speakers, but you should never be able to control the brakes over the internet.

In real life is that what actually happens? Sadly, no. Car companies seem to make a lot of bone-headed systems engineering design decisions (such as having a computer network path connecting infotainment to critical safety systems)