Is the iPhone made in China or the USA?

I'm curious, can you clarify something for me? When it comes to the iPhone, is it manufactured primarily in China or does it hail from the USA? I've heard differing opinions on this matter and I'm eager to get a definitive answer. I understand that the design and many of the components may originate from different parts of the world, but what about the actual assembly process? Where does the bulk of that take place? I'm genuinely interested in understanding the full picture.