Couldn't we have invented some way of doing all that cow-stomach stuff in a pot? Like cooking, grinding, fermenting etc, similar to how we turn other inedible plant stuff like wheat into yummy stuff like bread?

You can convert cellulose into glucose but it takes things like this (special cultured bacteria, extremely processed cellulose, and an ultrafilter to remove glucose to drive the reaction): https://onlinelibrary.wiley.com/doi/abs/10.1002/bit.26022080...

Or strong acids and 190°C over a catalyst.

Cellulose is remarkably strong, especially with lignin in the structure too. We're lucky anything can digest it at all.

Which brings me to mycology, probably the best way to extract energy from cellulose for humans. Sterilize, inoculate, keep moist, and harvest a significant percentage of the source material as good carbohydrates and protein.

My understanding is cows don't digest cellulose either. They have those stomachs to provide an environment for bacteria that can to live. Cows then digest that bacteria.

Which is digestion of course, but it isn't that cows can do it. Our large intestine does the same thing but it isn't anywhere near big enough to get a significant amount of energy from cellulose.

Seeing cows as bacteria eaters is a challenging but powerful image. Really clever point.