The Waymo agents are constantly monitoring the fleet for exceptions. When an exception is raised, a human reviews the footage and the situation. They can send commands to the car and suggestions about which way to branch or a course of action to choose. So ultimately, it is the Waymo Driver doing the actual driving.
If a Waymo gets really jammed up, they can indeed send in a human driver, who will appear in person, disengage the Waymo Driver automation, and drive the car normally. This has happened to me only once in 3,000 miles. The Waymo Driver, in my experience, is pretty good about keeping passengers informed when it's a bit delayed or confused by a complex situation. And the CSR will call in if they need to.
But no, it is a misconception that people have: Waymos cannot be "driven remotely" by any means.