Car manufacturers manage a ton of different outsourced suppliers. Yet still, they manufacture cars on their own.

Not always. You can have Magna Steyr develop and/or assemble cars for you. They’ve done so for a long time, for big brands like BMW and Audi too.

not all of them... many do outsource lower volume cars (usually luxury) to third parties.

"Magna currently holds contracts to produce vehicles for BMW, Daimler, Jaguar Land Rover and Toyota."

But is there any car manufacturer that fully outsourced manufacturing? It seems that owning factory is important to design cars.