Maybe I misunderstand what you mean but the car itself (chassis, bodywork, drivetrain) is a commodity today. If you can turn any car into an autonomous car then the car isn't worth that much in the self-driving game. Tesla's stock is propped by the promise of becoming the first to achieve autonomy not by its ability to build cars, or else VW and Toyota would be worth multiples of Tesla. They aren't because cars are a commodity. Autonomous car brains are the hot deal.
If manufacturing ever became Google's Achilles heel, they can take a majority stake in something like VinFast and manufacture a Google car.
It's like saying Linux will lose out to Microsoft when it comes to manufacturing. Yes, they do lose out by not manufacturing anything and yet their product is slowly taking over the market. Local loss (no profit from manufacturing), global win (core product is increasing market share).
They won't even have to (partially) buy any manufacturer - they can utilize contract manufacturers like Magna Steyr (which made the Jaguar I-Pace) or Foxconn, or lean on/expand partnerships like the one they have with Toyota.