Nvidia makes the GPUs and the basic card specifications. The board partners (EVGA, MSI, Gigabyte, Asus, etc.) make the final video card, the packaging, marketing, and distribution.
nVidia makes the base model, or reference design. Then they allow other manufacturers to take that reference design and add different cooling solutions to it, and overclock it to different specs.
Nvidia makes the GPU itself and sells them the other manufactures who then put it on a pcb and cooler of either their own design and/or they use Nvidia's reference design.
Nvidia makes the GPUs and the basic card specifications. The board partners (EVGA, MSI, Gigabyte, Asus, etc.) make the final video card, the packaging, marketing, and distribution.