Musicians

Composing doesn't mean that you're turning on the random number generator in your head and come up with something without any external input.

You will hear many melodies throughout your life. Your brain isn't a perfect storage device, but rather just mushes them together.
If your brain then reproduces mush between multiple songs and you write that down as sheet music, that's most the creativity you need.

You won't transcribe the mush perfectly and even if you did, it always sounds better in one's head than it does when written down. But then you clean that up, take out the rough edges, try to make it a bit richer than what your brain told you about and after a while you've genuinely got a thing of your own. Alternatively, you may also just need to scrap a snippet of mush, if it just does not work (which happens a lot).

Ultimately, perseverance and understanding the theory get you much farther than any creativity could.