As far as we know, Apple doesn’t own any of its manufacturing directly. The Mac Pro plant in Texas is operated by either Quanta or Flex[1], depending on who you ask.
You could argue that it’s largely a semantic difference, though, because those facilities are dedicated entirely to Apple and they’re usually the ones funding their construction.
1. https://www.cnbc.com/2019/09/23/apple-will-make-the-mac-pro-...
And does that really save on CapEx given that Apple funds the construction anyway and secrecy is absolute as well?
This helps keep crisp lines of delineation between design and production. That is my understanding.
The processes that Apple is outsourcing are considered disposable as the technology changes. The oral traditions and cultural knowledge are in the software/hardward/design ideals.
https://www.irishexaminer.com/news/arid-20333879.html
Otherwise, some repair/refurbishment lines might be Apple owned.
And if I recall correctly the last two generations of Mac Pro systems have been built in their own plants.
Of course that only says they have experience with making cases and motherboards, not down to the level of chip and screen components