Belly of Sheol
Here’s what it means
Jonah describes his near-death confinement and distress as being in the depths of the grave.
Why the Bible says it this way
An idiom is a familiar phrase whose meaning comes from the whole expression, not from reading each word by itself.
A helpful detail: Jonah was really inside the fish. The Sheol language expresses the death-like extremity of that experience and does not make the fish figurative.