Actually it’s way, way more complicated than that. The big automakers outsource most of their parts production. So any given part like a tail light or outside mirror is being manufactured by a third party. The automaker will place an order for say, a million tail lights to FOO manufacturing that specializes in tail lights. FOO manufacturing will then make 1.2 million of them and sell the extra 200,000 to auto parts stores, auto body shops, online retailers, etc. They’ll also retain the molds and templates necessary to make new ones should there be market demand.

Tesla makes all their own parts (mostly) and the market for aftermarket Tesla parts is quite small. This means there’s very little incentive for 3rd parties to step in and make Tesla parts. The side effect of this is that you can buy replacement parts for cars made by the big automakers for decades whereas parts for Teslas can become scarce once their production run is over.

Note: There’s a whole 3rd party market for unofficial parts/clones that I’m not going to go into. That’s where auto parts manufacturing and stocks get really complicated.