How does the human digestive system work step by step?

Hint: The human digestive system includes different organs that work together to carry out the process of digestion as well as absorption. Along with the organs, various enzymes also work in catalyzing the processes. The ingested food materials pass through these organs and after absorption of all the necessary nutrients from it, gets expelled out of the body.

Complete answer:
The process of digestion starts in the mouth or the buccal cavity which is the first part of the digestive system. The teeth and the tongue help in making the food material into a paste and mix it properly with the saliva. The saliva contains an enzyme known as amylase which initiates the digestion of starch.
The food materials then move into a tube-like organ known as the esophagus, which transports the food into the stomach. No digestive activity takes place in the esophagus.
The stomach walls release hydrochloric acid, pepsin and mucus. The pepsin enzyme helps in digesting the protein in the food, whereas hydrochloric acid adjusts the pH of the stomach for activating pepsin, and the mucous layer protects the acid from harming the stomach wall. There are multiple infoldings in the stomach wall known as Rugae that increase the surface area of absorption.
From the stomach, the contents move into the small intestine, where the major portion of digestion and absorption takes place. The intestinal walls secrete trypsin, chymotrypsin and carboxypeptidase that digest the protein contents, while the bile secreted by the liver digests the fat contents and the amylase enzyme secreted by pancreas digests the starch. The absorption from the small intestine takes place through numerous finger-like infoldings known as villi that increase the surface area for absorption into the bloodstream.
The large intestine is the next part where absorption of remaining water and certain medicines takes place. And the waste materials are formed.
The waste passes into the rectum and gets expelled through anus to the outside.

Figure: Human Digestive System

Note:
The walls of the digestive system are made of four layers. Namely serosa, muscularis, submucosa and mucosa from outside to inside. Serosa is the outer layer followed by muscularis made of smooth muscles that facilitate peristalsis. Followed by sub mucosa and later mucosa.