Partly home-made. Arm Holdings is British-based, but owned by Softbank Group (Japanese).
Apple licenses that and develops their own chip, which is then manufactured by TSMC.
So I guess if Intel dies the US will still have a few good CPU design firms, but no manufacturing
Also note that Foxconn (China) assembles the iPhones
Eg https://www.businessinsider.com/apple-iphone-factory-foxconn...
What you’re talking about is an ARM IP license, which allow the company to build their own implementation of the standard. Only a few companies have those and, of those, even fewer actually use it. Apple is one of those that does.
Plus Britain and Japan are both somewhere between close allies and client states. Nobody cares if we license from them.
Apple is near unique only in that they’ve pretty much never used reference implementations (since the PA Semi acquisition, at least) from ARM and stick to their pure bespoke microarchitectures. But they’re not the only company that could.