The control messages are transmitted via CAN bus

https://en.wikipedia.org/wiki/CAN_bus

The exact message content and protocols are reverse engineered.

Some cars use FlexRay instead of CAN bus but that has only experimental support.

Flexray is supported only on Audi on a specific Flexray-canbus integration board that was just end of lifed and Comma has no desire to support other boards. It was a proof of concept for anyone wishing to implement the same

Yea most modern cars have a Python API. https://github.com/commaai/opendbc

It requires a car with lane centering capabilities -i.e a module that commands the steering angle through canbus. Thats why not all cars are supported.