Waymo regularly brings the cars in to central service depots for cleaning and battery charging. Also, as a passenger you can report that a car is dirty and it will be pulled out of service for immediate cleaning (and you get a small credit towards your next ride).
Waymo has done a great job of making its rollouts into new cities smooth. I have to wonder what will happen once they pivot to profitability in a few years. Will they be willing to lose revenue pulling so many cars out of service? Will they optimize the human costs by not having so many humans on staff?
Maybe they'll kill Uber first and then drop the quality.