Few data points you might find interesting:
It is possible to (mostly) make a phone in the USA but not easy. Purism does it in low volumes with their Liberty Phone:
Introducing the – Liberty Phone by Purism
puri.sm
You'll notice it is somewhat lagging technology and they price the USA version ~ 2x the Made in China version.
The WSJ also did a parts breakdown of what goes into a modern phone like the iPhone. The parts were already sourced from across the world including the US. Here's the article but it's behind a paywall:
I suspect in a few years, TSMC will be able to make the latest Axx processors here (instead of Taiwan). RAM and storage is (or can) already be made here. The glass is made here but I suspect the displays will be tough -- the US lost that to South Korea and China so long ago it's unclear how long it would take the US to start making anything like the super-double WOLED++++ screens we've come to expect. Not sure about the camera assemblies. The rest of the things perhaps given a year or two...
But I suspect that constrained to a US-only supply chain, the cost of an iPhone will double and the technology will be somewhat lagging. But if you are content with running a basic phone for 8 years like I am then it may not be so bad.