Copyright attaches to the actual text (illustrations, etc; whatever is “fixed in a tangible form”), not the ideas.
You could write a story about a boy of humble origins who is whisked off to a special school, discovers he’s special, and fights evil. There aren’t that many original plots, after all....
You’ll only get into trouble if the main character is called Harry Potter of 10 Privett Drive, where he resides with his mother’s sister and her awful family, and he later attends Hogwarts, etc.
Copyright covers derivative works. If a story is plainly the same with names changed, the original copyright extends to it. This can be applied to the point of absurdity in music copyrights.
The derivative work has to be literally derived from the original "such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted." (per 17 USC 101). https://www.law.cornell.edu/uscode/text/17/101
The merger and scenes a faire doctrine permit lots of overlap in terms of themes, tropes and set dressing. You could certainly write a coming-of-age story set in a magical boarding school; indeed, Harry Potter is neither the first nor the last such novel. One of the classic cases is Walker v. Time Life. The two works, which were found to be non-infringing, both start with a double murder of two cops (one Black, one white) in the South Bronx, both feature demoralized Irish-American cops, and both have similar tropes (rats, cop-talk, etc). A reviewer might reasonably describe it as derivative (and a few did, I think), but not legally so.
What you probably couldn't do is publish the (non-parodic) story of Perry Hotter and his substantially-similar adventures at Pigworts, though that's also absolutely spoiling for a trademark fight.
Music gets weird because it a) seems like there are a lot of possible note sequences but b) there aren't really.
