Can't spoof gps all the time everywhere. Once you have a general idea where you are (usually because that's where you were when you stopped yesterday), you can compare what you can see with hd maps, and fix your position exactly. You know you're not in a building, but on a drivable surface for a start.

My 2006 car was fine driving underground for 10 mins with no gps signal, but got confused when I drove it onto a train and it moved 30 miles without the wheels turning. Then after a few minutes got a gps signal again and fixed itself. Had an option to manually set the position and heading too.

That isn't true. There were car navigation devices that predated non-military GPS. They used inertial sensors to understand the car's acceleration and turning, and used on-board maps to correct for the inevitable drift. The car made a 90 degree turn onto a side street but the map indicated that road was 30 feet ahead, it would reset its position to be 30 feet from where it thought it was.

I think the idea is that with a good idea of where you're starting, either by GPS or some other waypoints, you can avoid the need for GPS through dead reckoning and updating based on other known features. At least, that's my guess.

The first navigation devices didn't rely on GPS. I wouldn't be suprised if Waymo reintroduced this.

If you have high res images available of the vicinity ( and constantly update them ) you can work out where you are.