What's the difference between idioms and figure of speech?
A figure of speech can be made up on the spot. For example, any metaphor, simile, hyperbole, etc. is a figure of speech.
"The moon is like an irregularly-patterned soccer ball".
That is a figure of speech. But it's not an idiom because I just made it up.
If people catch on to a certain phrase, and it becomes common in the language, it becomes an idiom.