How the iPhone 6 is made: Infographic
You may think the main reason iPhone 6 is still assembled mostly in China is because production costs there are really low. It turns out, one of the main reasons behind that is actually scalability not price (US-assembled iPhone would cost only $4 more). In the US setting up an assembly line would take 9 months while in China you can do that in merely two weeks!
Thank you to Compare Camp and Alex Hillsberg for providing this data.