That depends entirely on what kind of part you're talking about.

Any kind of metal part is probably going to be outside your reach for desktop manufactury. There are certainly industrial commercial 3D printing providers who are willing to do one off manufactury, but it's not cheap.

If you're talking about simple plastic pieces, then most designs and models are made by individuals. Usually in some sort of 3D parametric CAD system (like Fusion 360 or Onshape), though you can use photogrammetry to take a series of carefully chosen photographs and have the system build a proper 3D model.

For the most part, auto part 3D models aren't available from even the manufacturers at this point. That may be something that happens in the future, but I wouldn't bet on it. When an industry makes money selling parts, they are really unlikely to want to help people make those parts for themselves.