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.

Dell desktop workstations have custom cases and mobos and power supplies, at the very least. Laptops are custom cases and often SoC.