Adding a vehicle is the equivalent to writing drivers (heh) for your particular car. It entails reverse engineering your car's builtin driver assist features to determine which CAN messages do things like actuate the brakes and gas, turn the wheel with a given amount of torque, or communicate radar values from the cars builtin radar sensor.
For many cars the CAN messages will be the same as similar models so porting a new vehicle just means adding tuning values that can be captured by driving with OpenPilot connected but not controlling the vehicle e.g.: https://github.com/commaai/openpilot/pull/866
For many cars the CAN messages will be the same as similar models so porting a new vehicle just means adding tuning values that can be captured by driving with OpenPilot connected but not controlling the vehicle e.g.: https://github.com/commaai/openpilot/pull/866