The laws are more laws of economics. Airlines like to be relatively vertically integrated, especially at their home bases.
Absolutely not. Read the Vanity Fair article I linked above; they outsource it to China, El Salvador, etc. where labor is cheap and non-unionized and the FAA can't meaningfully do surprise inspections.
The airline will have approved routine maintenance and repair procedures from Boeing, and reps they can reach out to with questions. When it's a major or unusual repair, they can ask Boeing to handle it; https://www.smithsonianmag.com/air-space-magazine/airliner-r...
> I would expect the aircraft vendor to control everything.
It's not dissimilar from cars; most people don't go to the dealership for every repair, because it costs substantially more. Especially not if the plane is based in Senegal.