@Dan Roberts Probably the same way every car with stability control made in the last 20 years does, and ones with lane assist - There's a steering wheel position sensor that measures exactly where it's turned to, and the computer does the magic based on that and the gyro or yaw sensors. If it needs to it can nudge the steering wheel or provide input such as when lane following. It also cuts power and applies brakes as needed
There's some legal and safety issues to overcome before going drive by wire for steering. Car companies have been wanting to switch to joystick steering for decades but they can't due to safety concerns in the event of a failure. Same with brakes which still have a manual component. In airplanes they use fly by wire fairly often.
That will be interesting to see how they institute torque vectoring and adjusting power to the wheels to compensate for drift without drive by wire.