Earlier quoted context omitted.
Engineering Explained did a really good video about this: https://www.youtube.com/watch?v=agMrewRJTow
To save going through that whole video for the answer: they have redundant power supplies (3 batteries total) and redundant actuator motors. But the fact is if there is a bad enough electrical/wiring fault, especially as the car gets older, you are toast and you have no steering. Of course the same thing can happen with mechanical steering (eg a broken linkage) it comes down to the quality of the design and build.
Probably won't even allow you to start the car if one of them is faulty.
This kind of thing will almost certainly be designed to comply with ISO 26262.