I don't know about Waymo specifically, but I worked for a competitor for a while and the AI driving the car is not a large language model. It isn't trained on stories or even words. It's whole flow is: given this sensor (camera, lidar, map, etc.) input, generate the "best" control (steering, brakes, acceleration, etc.) output. Capture input and generate output at a rate of say, 30 times per second (or whatever rate they have gotten to now).

The training data for this comes from trained, careful human drivers. And the whole AI control loop is run in conjunction with a more deterministic system with safeguards for cases where the AI perhaps decides to steer towards a tree. There's also provisions for uncertainty. If the system isn't confident enough in what to do based on the given inputs it will switch to a safe stop mode and call a human up for help.