You are entirely incorrect. SpaceX perfected friction stir welding for large sheets of aluminum for their fuel tanks. That tech was
transferred that technology entirely to Tesla, who happen to make cars from large pieces of aluminum (Model X and S).
https://electrek.co/2015/05/24/spacex-transferred-novel-weld...
There is no way that they are using friction stir welding in a high volume line. It's a very slow process, visually it's horrible (big scar along the joint) and, though I've no experience with it I would have thought that pushing a spinning piece of metal at high temperatures through two thin abutting pressings would be highly unreliable.
"though I've no experience with it" - aka you're not qualified to answer whatsoever. At least you're honest in your trolling :)
SpaceX did not invent friction stir welding, but they did develop a mastery of the technique. Luckily, unlike you, they hire people who have experience in it. The strength of friction stir welds is one of the reasons they use it in the Model X and Model S vehicles, both which blew it out of the park regarding NTHSA safety standards.
Musk himself describes it here (for SpaceX):
https://www.wired.com/2012/10/ff-elon-musk-qa/
They even mention a few of the aerospace bits they have in the Model S:
https://www.tesla.com/blog/tesla-model-s-achieves-best-safet...
The key isn't being able to do it (as someone else said joining metals is a solved problem), it's being able to do it with high quality and in a reliable way. Do you think Space X joining techniques are optimised for making 100k joints in a factory environment?