I got the head unit changed in my Subaru . In the Subaru, the systems that the Head Unit controls (entertainment system, backup cam, Sat NAV) are totally separate from critical systems. So everything the head unit controls can fail and the car will still work.

And you’re right, there’s an interface/translation layer . In my case the guy used an iDataLink Maestro. It seems the only thing the iDataLink company does provide interfaces that allow head units to talk to car computers, so that the Head unit manufacturers don’t have to bother.

I’m not sure such separation is common in EVs.