tl;dr – they turn Nvidia's product (the GPU, memory configuration, and chip design) into the thing that consumers buy (the actual graphics card you slot into your PC).
They're what's called an AIB (stands for Add In Board). They buy the actual GPU chips from Nvidia (or AMD, for other companies) and manufacture their own graphics cards.
For some cards, an AIB might design their own PCB, or they might license the "reference" design from Nvidia. They'll also engineer their own cooling solution.
Often, an AIB will offer several different variants based on the same GPU. For example, EVGA offers a few different models of Nvidia's 3080 GPU – mainly their XC3 and FTW3 models. The FTW version is a little more expensive, but has a larger heatsink, better power delivery, etc. The XC version will probably run hotter and louder, but can fit in smaller cases. They're both "RTX 3080"s, but EVGA has built 2 distinct products (actually way more than 2, in real life) from the same Nvidia GPU.
