Does the process consist of more than a particular microbiome that can process cellulose?
Humans are hindgut fermenters. We are not as good as it as herbivores, but we still do it. Our colon exists to do hindgut fermentation.
We are bad at it because we have a small colon and don’t keep the food there very long. We still get some short-chain fatty acids from it and vitamins.
Hindgut fermentation is an evolutionary adaptation for animals that eat a lot of cellulose. Humans, and our primate cousins, are omnivores. There was no need to evolve the ability to digest cellulose because we can get all the nutrients we need from other food sources