Quoted from ZenTron:iPhone was designed and engineered by Apple in the USA which is #5 in Fortune 500. The parts are all sourced and supervised by Apple. All thats being done in China is putting the things together aka the manufacturing in buildings which have suicide nets wrapped around them.
Companies like Foxconn are not just assemblers. Be it Eastern Europe or Asia, these companies exist to not just assemble stuff, but help companies industrialize their products, make it producible, drive costs down through sourcing and optimize their designs. They are full end to end shops from design, sourcing, building, packing, warehousing, and logistics.
While you can simply contract out 'factory work' - that is not the limit of what these major companies do.. and nearly all companies rely on their production partner to help optimize their products for manufacturing.
Apple may come up with an idea - but its the Foxconns of the world that work to make it a reality that can be built by the millions.