They're not. Use any car's heads up display and you can configure an enormous number of things. Even if there was somehow a pure separation, things such as "playing video while the car is moving" are regulated in many jurisdictions and would land firmly in the "UI" layer.

You'd hope so but I fear that many safety critical aspects run on the same system as the infotainment system... And that's a perfect excuse for manufacturers to keep these things completely closed