Is Apple big enough that they could commission their personal chip factory? So it would be co-owned with a major chip fab but Apple can choose to buy all of it's output or agree to have some output sold off to other customers
Not practical. If you want to know more about how the economics of these sorts of big industrial developments work, the 'Asianometry' YouTube channel does great work summarizing what goes on.
Apple is an American company. If they were to do a real partnership in a factory in China, where it would make sense, that would be politically bad for them.
So, they can only do partnerships in the US. That's obviously going to be a joke.... they supposedly have some deal going with Samsung to produce well-behind-the-state-of-art chips for Apple Watches in the US, but that's just spending money on PR in the same way that their sponsorship of the Trump Ballroom is.
And outside of just sponsoring a project, they definitely would never practically benefit from building their own chip fabs. Look at CXMT for example. They are 'only' doing DRAM but it still took a decade of recruiting/R&D/factory building to be able to build mid range DDR5 chips on a 15nm node.
A company like Apple could never justify spending tens, hundreds of billions just to
hopefully have more stable memory prices ten years from now. After the AI bubble has burst and there's no money in memory again.
And there's no world in which they would benefit from trying to chase the state of the art nodes which their actual processors are manufactured in. They could spend hundreds of billions for 15 years and at that point they still might not be up there with where Samsung and Intel and whatever racially superior Chinese competitors arise will be at that point.