As I understand it, the majority of individual smartphone components would be manufactured using automation, and the primary step that takes place in Shenzhen is someone physically putting all the pieces together. I couldn't make an informed comment on the feasibility or cost effectiveness of trying to automate this process as well (I'd guess it's fairly low due to the relative uniqueness of each phone model) but maybe someone else can.

And Tesla is well behind the major car manufacturers in using automation.