It takes a decade to build up a supply chain for complex electronics products. Even though Apple, Foxconn, Wistron have started building in India, it's going to be a while before it's able to service the capacity that China is at and even longer before margins (both quality and profit) are up to where they are with their Chinese lines (which are more like city sized factories with tons of supporting infrastructure).

Yeah, Samsung somewhat recently started assembling phones in India, but they are a lot lower build quality compared to their Vietnamese or SK made units. Probably within a year or two India factory will catch up on quality, but unlike Samsung (who have a lot of budget models) Apple can't afford to ship internationally bad quality iPhones for a couple of years.

What's the source for lower build quality of Samsung's Indian made handsets?

Personal experience.

There is also this thing that working in Foxconn factories is apparently very though. And pushing the problem to third-parties seems to be very convenient in terms of reputation and salaries/contract.

Isn't the biggest problem with iPhone production that the volume is so high that very few places aside from China have the requisite number of engineers to build such a plant?

Remember hearing about Detroit back in the day? Where the transmission was made down the street from the auto factory. You probably didn't need to go more than 20 miles from the auto factory in Detroit to get 90% of the car parts. If one supplier had a problem, you'd just go to another that was only slightly more distant.

Shenzen is like that. Everything you need for electronics manufacturing of nearly anything is located in the city. This means that if part of the supply chain is disrupted in normal times, you could go get the same part from somewhere else not far away.

Today, the United States auto manufacturing isn't that way... and frankly it isn't even fair to say "United States auto manufacturing" - https://cimg3.ibsrv.net/gimg/www.corvetteforum.com-vbulletin...

So, here's the problem that Anyone building an iPhone would have - where do you get the capacitors on the board from? how much lead time do you need to keep the supply going? What happens if the supplier isn't able to send them? How big of an industrial base do you need to make all the parts and how distributed is that industrial base?

But the iPhones aren't assembled anywhere near Shenzen? The article mentions Zhengzhou which is about halfway up China (where Shenzen is at the southern edge).