That "open stuff" generally speaking is either an ODB-II port (which as I understand it is generally limited in diagnostics and communication in modern cars) and the other is the C2 Radio harness (which plugs into the back of the infotainment system.) These systems exist on all modern cars.

The C2 radio port gets you direct access to the vehicle's CAN-Bus system, which itself is generally segmented into a high frequency (safety critical systems: drivetrain, drive-by-wire steering if applicable, braking, etc) and a low frequency system (door locks, sound system, windows, etc.) If you reverse engineer your vehicle's CAN protocol (each manufacturer uses their own codes on a standard CAN transport protocol), you can issue commands to it directly from your Pi.

Tesla. I probably don’t have to say much more.

Here’s an older video with some interesting details: https://m.youtube.com/watch?v=KX_0c9R4Fng