If/when iPhone manufacturing comes to the US, it's going to be massively automated, with a comparative handful of employees to oversee the robotic assembly lines. Leaving aside the labor cost issue (yeah, Florida and some other places would like to put kids to work), the US can't come up with several hundred thousand people who all want to move to one densely packed area to do manufacturing (close tolerance, detailed, skilled, but repetitive work) in a collection of monster-sized production plants, like they have in China.The US won’t find the cheap labor that they have in China (for good reasons I’d argue, although Florida seems cool with removing child labor laws so maybe I’m wrong). So prices will go up if all iPhones sold in the US are built in the US.
I heard a bit on a podcast the other day, reporting some survey results, and lots of Americans want to see manufacturing come back to the USA, but when you ask the same people if they would take one of those manufacturing jobs, suddenly it's a different story - none of them do. They want the industry here, but they don't want to be involved (other than buying the products). So, yeah, building a new densely packed city with a quarter million people all working in the iPhone parts factories isn't going to work. (And if it somehow did, it'd take years or decades to set up, and maybe then you'd want to turn on some tariffs temporarily, to give them a boost - not now.)