Someone wrote here that man cannot digest cellulose.  This was widely taught
in medical schools in the 1930's, but every veterinary student knew even
then that was false.

Here's  the griff:

True obligatory carnivores cannot, hence dog and cat foods that contain COHs
are a scam.

Omnivores and herbivores can and do.  They harbor anaerobic bacteria and
other wiggly-wogglies ('microrgs') in their gut, which do the digesting for
them.

Those break cellulose into their constituent simpler sugars, which are then
absorbed through the gut wall in the usual way.

There are two main systems.  The simpler is caecal.  The mainly cellulosic
residues that get down to the colon after the protein components have been
digested are metabolised by the caecal microrgs.  They are broken down into
mainly simpler sugars, methane, and a little hydrogen.  The gases are partly
absorbed and exhaled in the breath, and mainly expelled anally: farted.

The other main system is digastric.  The herbivore stomach is divided into
several compartments.  The partly chomped food goes into the first, and
loosely available sugars and proteins are digested and absorbed.  The
partially digested residues are regurgitated into the mouth, and chewed
thoroughly, chewing the cud.  They are then swallowed again to another
stomach compartment and then on down.  The gases are expelled from the
mouth, by belching.

Tropical explorers who report that they heard a herd of elephants from afar
by their farting are misinformed;  they were belching sonorously.

The important distinction between the two systems is that the caecal
digestors lose all the protein content of the microrgs in their faeces;  the
digastrics absorb those in their duodenum, and so benefit from their greater
overall nitrogen absorbtion efficiency.

This is a very compressed description, but it is a start.  If you want to
know more of the details, consult a veterinary textbook, not a medical.

Geoff