Here's an explanation of CarPlay Ultra, which really is the phone driving your dash and instrumentation: https://www.apple.com/newsroom/2025/05/carplay-ultra-the-nex...

There's only a couple of Aston Martin cars that support it, but there's supposed to be more coming. See: https://www.stuff.tv/features/apple-carplay-ultra-compatibil...

What year do you live in?

The "infotainment" (the display on the middle of the car) controls a lot of things now. On my car I can change things like lane departure or collision warning, changing the car's behaviour, for example that it brakes itself if it thinks it's about to hit something. You can even set a speed limiter, changing the behaviour of the car when you put pedal to the metal. Instead of going faster and faster up to 220km/h, you can modify the car to go faster and faster, but only to e.g. 120 km/h. How's that "external to the car".

You are mistaken, sorry. Even the normal everyday infotainment on most modern cars has receive-only telemetry available to it from the car control computers. Some elements are passed through (like EV state of charge) to old style CarPlay as well. Think data diode (making no claims as to the actual physical implementation, but logically it works)

CarPlay Ultra is just an extension of that where the gauge cluster is now just another infotainment screen that displays the received telemetry data. It does not have access to the ECU, cannot interfere, can be rebooted with impunity, etc.

Wouldn’t that mean CarPlay Ultra is flat out impossible?

But it exists. And is available in at least one production car.