Clayton Christensen has written and talked about how Dell sold their business piece-by-piece. Each individual step seemed like a good move - it lowered costs and increased profits. Until one day the companies they outsourced to had it all and they started selling better computers for less money than Dell.
Speaking of their servers, their motherboards and chassis seem to be of their own design. Often times these layouts are pretty proprietary and designed exactly around the design of the chassis. They do some level of customization of firmware on the devices they go with, as there's usually Dell firmware on things like hard drives which integrates more with their own management tools. Arguably their software stack of actually managing fleets of their hardware is something they design, along with the actual remote access controller systems they put on the boards.
