I don’t understand your comment. Apple chose a single source so by definition, there is only one manufacturer.
My point is that the process is highly transportable unlike a precision mechanical assembly where you can have very long lead time items like complicated robotic assembly lines, and manufacturing processes that require significant knowledge transfer when they move.
Pretty much every contract electronics manufacturer in the world is already set up to stack a bunch of parts into a case and to an end of line test. The Tooling (bills of material, PCB artwork etc) can be transferred in an email. The gating item in setting up another source for iPads would probably be component lead times.
There is the problem of all those CNC milling machines for the aluminum cases. Not many electronics manufacturers have those set up for production. I believe Apple's parts makers such as Foxconn are the only ones using it on such a large scale. And with Apple's investment they bought up much of the world production of such machines for a while, so it would be hard for anyone else to get up to speed very quickly.
My comment was based on your assertion that the iPad is made of parts that can be assembled by anybody... it is NOT. Apple subsidized the machines that make it... and the machines that make the iPhone as well. There are very specific processes that go into making the iPhone that other makers DO NOT USE... Apple fronted the money to make those machines that are installed at the factories that make those products. Other assemblers do not have the ability to do the job, because the investment in the proper machinery has not been made. Had you been following the development of these products you would know this. There also is not just one country of manufacture. There are at least two. iPads and iPhones are manufactured both in China and Brazil. There are also at least THREE contract manufacturers of iPads and iPhones, not one, all with equipment subsidized by Apple.