From what I've read, the difficulty is largely in coordinating production of various different components. Apple not being able to build macs in US because they couldn't source enough of a particular type of screw is a perfect example of this. Every device like a phone has thousands or millions of different components needed to assemble it. Auditing the whole supply chain to ensure that you can produce all of these components domestically and in sufficient numbers is pretty difficult. China has a huge advantage here because it's where most advanced manufacturing is happening.