Nearly any line is jagged and a wide swath of area if you look at it closely enough. Now, I’m not a lawyer, but I do watch some on youtube… that makes me qualified, right?
I’ll ignore the outright plagiarism you’re noting above and assume US copyright law based on what I’ve seen for my youtube law degree. Fanfiction is part source material and part new material. Author(s) of the source have copyright interests in their own works *AND* that part of your fanfiction that leverages their material. However, the fanfic writer does have copyright interest in the material they added in crafting their fanfic. Fanfic writers usually slap on a disclaimer, disavow profit, seek fair use as grounds for posting, and call it a wrap, easy done, easy go. BUT unless that fanfic writer has signed it away, they’ve got copyright in what they contributed, and therefore, they can use the DMCA against the offending website. That’s my $0.02 on how a fanfic writer can deal with a website that pirates their stories. If they want to go further, if there’s a legal tassle, then I’d seriously recommend consulting a copyright lawyer who can evaluate the situation.