The thing to understand about copyright is it is never black & white, well almost never.
Anyone can sue anyone for anything it doesn’t mean you’ll win.
There really are only so many ideas out there. Then they start repeating.
An example from the Simpsons is Lisa tells a story about the Seven Dwarfs, and immediately a lawyer from Disney shows up and threatens a lawsuit. Lisa replies “you can’t copyright fairytales, they belong to everyone.” Well she’s mostly correct.
Snow White isn’t subject to copyright, but Disney’s version is subject to copyright as are the Dwarves names. The original fairytale didn’t name the dwarves, so you couldn’t use those name, but you could give them new names and personalities.
Now how does one decided how much of an idea you can take before it stops being an original work and starts being an infringment of copyright?
That is the problem, you can’t always tell, till you publish the work and someone sues you.
So who decides what IS and what ISN’T a violation? A judge does, and this is why copyright can be so “iffy.” One judge can decide it one way and another will decide it another way.
Also remember copyright needs to be enforced by the owner as well.
A classic example of this is the Honeymooners versus *the Flintstones
I read a bio of Jackie Gleason and in it he says he was often asked why he allowed the Flintstones to go on when it was such a blatant violation of his work, the Honeymooners
He replied that he couldn’t do much except sue and have the show taken off the air. He then explains he’d be known as the man who killed Fred Flintstone and he’d have every child and their parents hate him. And then they wouldn’t watch HIS show.
So as you can see copyright is not always as cut and dry thing, even in seemingly cut and dry cases