IIRC, they won't sell parts to a non-certified mechanic, and there's a lack of third-parties making the more complex parts. You're fine if you need a tire swap. You're not gonna have much fun if you have to replace the screen.
A Federal-level right to repair can't come soon enough.
You can buy (almost?) any part directly from Tesla as a plain Joe [1], but you may need a VIN for certain parts (mainly the actual computers?).
In the event you want to DIY a repair, Tesla publishes all their service manuals they use at their service centers (e.g. [2]) and you can can even use the exact same software their technicians use for the deeper repairs, albeit at a price that is expensive if you're a plain Joe, but for a repair shop it doesn't seem to be terribly expensive.
On the topic of 3rd party parts, there isn't quite as robust a marketplace.
Nothing. But they don't need to. They just tackle it at the supply side.
If you're a mechanic who wants Tesla parts, you need to go through Tesla. If you go through Telsa, you can't use third-party parts or resell the first-party ones. As a result, the market for third-party parts stays largely too small to exist.
A Federal-level right to repair can't come soon enough.