Car manufacturers make very few "parts" themselves. Gearboxes, as an example, are often designed and manufactured by companies like ZF[0] or Valeo[1]. They also sell these to the public at (usually significantly) elevated costs. I'm sure this could be done for specific Apple parts, but like the car manufacturers, Apple will want some control over the parts they design.

[0] https://www.zf.com/ [1] https://www.valeo.com