What kind of manufacturing plant is the question. Competing with Intel and a processor design and fab house is obviously beyond all but the largest companies. I suspect even designing a motherboard is beyond a company of their size (or even an order of magnitude larger). However it probably makes sense for them to do final assembly so they can integrate quality assurance steps where required. Take a case somebody else made (their design), a bunch of other components (mostly off the shelf, but some custom), and screw them together. At several points along the way power up the system and run some tests.