iPhone is a high integration model. Like said before, suppliers are located around the worlds. They separately work on their own components contract. Most components are produced in Asia, while some other in the US and Europe.
When components are completed, they are exported to assembly facilities from worldwide suppliers.
Export Route (From Asian main supplier to mainland China):
Main components of the iPhone like displays, camera lenses and microprocessors are mostly produced by Asian companies, including those from Taiwan, Japan and South Korea. Because of these products produced and exported, it is estimated that Japan’s electronics exports can raise by 5% while Taiwan’s electronics exports can raise by more than 8%. If people move their sight more forward, they can easily find that Asian economy get boost from IPhone. Thanks to large demand from Apple, some of these main suppliers and other US-based companies outsource fabrication to other Asian countries with cheaper labor or greater ability to meet the large scale of production.