Digestion

After the common and chemically simple dietary components like proteins, fats, and carbohydrates are digested and absorbed in the small intestines, dietary fiber, which consists of plant polysaccharides and other undigested material, moves into the large intestines.
Light & Nagler 2024 Immunol Rev 326: 203-218

The large intestines are home to hundreds of bacterial species. These bacteria encode thousands of enzymes. These enzymes enable them to digest the remaining contents.
Boutard et al. 2014 PLoS Genet 10(11): e1004773
Lapébie et al. 2019 Nat. Commun. 10: 2043

The gut bacterial microbiome has an impressive digestive capacity. This is evident from the fact that gut bacteria collectively encode about 1000 times more polysaccharide-degrading digestive enzymes than humans do.
Kaoutari et al. 2013 Nat. Rev. Microbiol. 11: 497-504

The hormones gastrin, histamine, CCK (secretion of digestive agents), motilin, 5-HT (motility), GLP-1 (transporter induction), somatostatin, and secretin are involved in digestion. They affect the secretion of digestive agents (acid, hormones), motility, and induction of transporters.
Fothergill & Furness 2018 Histochem Cell Biol 150(6): 693-702

see also:
Glycan-degrading carbohydrate-active enzymes (CAZymes) / CAZy