Search for "driver monitoring system". As soon as there is something unexpected there is an actual driver taking care of it. Also the reason why it's not level 5, but stuck at level 4 self driving.
And they're not drivers, they're supervisors. They don't take over driving, AFAICT they issue high-level commands like "safe to proceed", "take the right lane", et cetera.
I mean, this is true for Uber as well.