How do we understand the Book of Jonah? Was he really swallowed by a whale/fish/something else? How did he survive for three days in whatever it was? Is this just one of those things we must just understand as a miracle?
According to the writer of the Book of Jonah, when Jonah was thrown into the sea he was swallowed by a great fish. This could have been an extremely large fish, or, more likely a whale. Again, according to the Book of Jonah, “Jonah was inside the fish for three days and three nights.” Obviously, no one can prove that this happened, but I am a Christian and I am convinced of the inspiration and reliability of the Bible, because the evidence supports this conclusion. I also believe that God can and has worked miracles. For these reasons, I believe that the story of Jonah being swallowed by a huge fish and surviving is true. Some have tried to give natural explanations for what happened to Jonah. To me, this does not make sense. What happened to Jonah was obviously a miracle. It is presented as a miracle in the Bible, which means that no natural explanation will be correct. I am of a minority opinion here, but I believe that what is most likely is that Jonah died in the big fish and that God raised him from the dead on the third day. Otherwise, how can we explain that it took Jonah three days to repent (Jonah 2:1)? This is somewhat speculative on my part, so you will have to decide what you think.
So, my answer is Jonah was swallowed by a huge fish or a whale and that his survival was an unqualified miracle. The exact details of the nature of that miracle I am not sure about.