As with all animals digestion in fish involves the breakdown of eaten food into its smaller component parts, amino acids, vitamins, fatty acids etc. which can then be used to build up new fish body. The breaking apart or breaking down of the eaten material is called anabolism, the building up of new material is called catabolism and these two together make up the whole of metabolism. Grammatically it follows from this that the respective adjectives are anabolic, catabolic and metabolic.
When compared to humans, the digestive system in fish is relatively simple. There will of course be variations that are species-dependent. Some species of fish have teeth. The teeth in fish are generally adapted for performing special functions. Predatory Catfish will for instance have small sharp pointed teeth, while the Pacu fish have teeth adapted to crushing fruits and nuts.
The Big fish that swallowed Jonah is often said to be a whale, however the Bible does not make it clear at all. It was a big fish. A fish could be large enough to swallow a human. After some rudimentary searching and research on the web we can illiminate most species of fish because they rarely exceed 2m. In order for them to swallow a man whole they would need to be all stomach and we know this is not the way fish work.
The largest fish known to man is the whale shark and that could probably fit a man in it's stomach, Also the basking shark has the right kind of dimensions but both of these beasts eat plankton and small fish. Could they swallow a human whole? According to the Smithsonian the Whale sharks Oesophagus is only inches across so this is wholly unlikely. In experiments with whale sharks rice was thrown into their mouths and was immediately spat out and the same for some seaweed. We can assume that a human would be spat out too as the whale shark would know it cannot swallow it.
There have been reports of Great White Sharks and Groupers large enough to swallow a man. It is also posited that a sperm whale could swallow a man. So far the tale of Jonah has a few creatures large enough to ingest a full sized man. But let's find out what would happen to a man swallowed by one of these beasts.
The Sperm Whale like a Cow has four stomachs. That is a lot of digestive juices to survive, but the biggest problem with this scenario is the lack of oxygen in a sperm whales digestive tract. 5 minutes without oxygen is enough to kill a man so we can discount the Sperm Whale as a suitable taxi for Jonah.
During a fishing competition in Australia, a Great White swallowed whole, a 2.1m Dusky Shark of 80kg which was hanging at the stern of a boat. THis proves that a man could be swallowed whole as well. Most sharks swallow their food whole or bite it into relatively large pieces. Sharks have U-shaped stomachs that use very strong acids and enzymes to dissolve most of what is eaten. The stomach produces an easily absorbed, soupy mush. Only this liquid mush enters the intestines because the pyloric valve (the valve between the stomach and the intestines) is small. Indigestible things, (like very large bones and non-nutritive items) are vomited. I guess going in is easy coming out less so.
Groupers are alleged to be man eating but I cannot find any reports of a man being swallowed whole by a grouper, Reports of arms being sucked into groupers mouths and divers being able to extricate themselves, I don't have to tell you that being swallowed by a grouper would kill you anyway, do I?