Yes I'm aware of the contract manufacturing that Foxconn(among others) does for Apple. I'm also aware of the ease of doing business in China(as opposed to other countries which also have cheap labor) mainly bcuz as u mentioned there's a whole ecosystem in place to do so. This is why countries like India and Vietnam haven't had as much success as they had hoped when trying to lure manufacturing away from China. The same applies to countries like Pakistan, Afghanistan, Bangladesh, Sri Lanka, and others that have cheap labor.Apple does not manufacture mobile phones by itself. It orders Foxconn (and other Taiwanese and Chinese companies) to assemble the phones with components from many suppliers. Companies from various countries, including Vietnam or Pakistan, are doing the same. Apple is just the biggest customer of Foxconn.
Second point is that there are many component suppliers in China and they can manufacture nearly 100% quality components at relatively good price, from screw to screen to processor. And there are perhaps several hundred, if not thousand, of such components in one mobile phone
In the meantime, to be honest, Vietnamese suppliers cannot manufacture even a simple screw to be used in the phones, although we have the biggest assembly plant of Samsung outside Korea. Technically we can manufacture the screw (And any African country can do as well), but we cannot do it at a reasonable price (no technology (imported), no suitable metallurgy (imported), lack of skilled manpower (very high salary). Not to mention more sophisticated components like screen, processor, memory chip etc.
That is the difference between China and other countries, which are basically not related to the labor costs. And you can list other advantages like excellent infrastructure and logistics.
P.S. I don't mean to demean what u wrote...it is very informative and I thank you for it. It just so happens by chance that I have already read up enough on it prior to u posting it.