There's a simple test I find useful to determine who's driving:

If the vehicle has a collision, who's ultimately responsible? That person (or computer) is the driver.

If a Waymo hits a pole for example, the software has a bug. It wasn't the responsibility of a remote assistant to monitor the environment in real time and prevent the accident, so we call the computer the driver.

If we put a safety driver in the seat and run the same software that hits the same pole, it was the human who didn't meet their responsibility to prevent the accident. Therefore, they're the driver.