The domestic vs. offshore production calculus is actually closer than a lot of people realize, in many instances. Often it's a choice between a highly-automated and capital-intensive production process in the US, vs. a manual process at a contractor in China. The combination of higher shipping and very low cost of capital in the US is a pretty good one.
Whenever someone tells you that it's impossible for the US to rebuild its manufacturing base, go take a look at factories in China: they're not that old (esp. the light industrial stuff). What was built in China in 20 years could be built in the US in the same amount of time or less, if the economic case was there. There's nothing magic about China, it's just a place where labor happened to be cheap at one point in time.
The US doesn't manufacture those items because it has no capability to do so.
There are no factories, no supply chain, no expertise and no known how.
There are some things that are cheaper to manufacture in US, (surprise!) if you manufacture for the local market, than in China, but them being simply cheaper still doesn't mean you can actually manufacture them.
I myself tried to setup an electric scooter factory, first in Canada, and than in Washington state. We were simply unable to find even moderately competent manufacturing engineers, nor the line workforce.
Of course, every consumable and every part had to be imported.
And both of the above meant that we decided to close the business in under 6 months.
https://www.kut.org/post/one-reason-apples-computers-are-mos...
https://www.nytimes.com/2019/01/28/technology/iphones-apple-...
Basically, the US simply does not have the infrastructure to manufacture complex items in mass quantities. We haven't invested in that infrastructure, whereas China has, so they have the ability to do this stuff, and we don't. So trying to build an iPhone in America is like trying to build an iPhone in Zimbabwe or Sudan or Armenia: the infrastructure simply doesn't exist and it isn't feasible to take on a project like that there.
America stopped being an industrial powerhouse decades ago, by choice, and you can't just bring that back overnight, especially when some other countries never gave that up and will easily out-compete you.
Nearly all the supply chains are in Asia. It would cost companies billions to relocate and they would just relocate to other areas where the labor is cheaper and they can get away with working their employees 12 hour shifts. Things like electronics for example would be extremely difficult to switch to a country like the US because there isn't a trained workforce here to deal with manufacturing small components. Plus, they'd be asking to be paid 20 dollars an hour with benefits vs China's probably 12 dollars and no benefits.
Also, leaving the UPU is as drastic as the tariff war because all it will do is cause countries to jack up the rates of incoming mail to compensate. Every time the US leaves without negotiation, there's no reason why countries won't retaliate.