Back in the day, I wrote fanfic. I was working on one for "Lucifer", but I kind of lost interest in it. I enjoy reading it, though.

But first I want to mention that the YouTuber in the OP is peddling a writing tool, ProWritingAid, that uses generative AI, which I'm very much opposed to and think harms writers and writing. I've posted a number of OPs about that over the last few years.

Re fanfic - to be honest, I've never understood anyone wanting to write or read it. I think it's least objectionable if someone wants to utilize a professional writer's characters to write something just for personal writing practice, and then never show it to anyone or maybe show it to a few friends, but not publish it, online or off, and not try to get lots of attention for it.

The best way to show how an artist you admire has inspired you is to express your own creativity, and not copy their creations and try to live in their fictional world, controlling the characters they created as if they're your puppets.

Some professional writers don't mind fanfic. Others do, and IMO people who are real fans of an author who doesn't want their work used for fanfic should always respect the writer's wishes. Especially if that writer is continuing to use a fictional universe and characters. Trespassing in that fictional universe to throw fan-created plots and relationships all over it could too easily pre-empt plots and relationships the author had planned for future stories. And that's much more of a problem with fanfic published online.

If you wouldn't interrupt a friend's long, interesting story about their family's overseas vacation to tell everyone listening how you imagine the rest of it went - or if you wouldn't invite yourself to move into a friend's house, bring some of your own furniture and rearrange theirs, and tell your friend's family and friends how they should get along and what they should say to one another - you should stay off an author's intellectual property. No matter how much you love it.

I know some fanfic writers later became pro writers, though the only one that comes to mind at the moment is the woman who wrote fanfic based on the Twilight series, and turned some of that into Fifty Shades Of Grey (zero interest in either, so I have no idea how good a writer she is).

What I said about fanfic applies to music, too. Being influenced/inspired by great artists is wonderful. Copying them too closely doesn't show as much respect as stretching your own creative wings and later giving them credit as an influence.

And fanfic about musicians inventing relationships for them is simply creepy.

So are AI videos some fans of musicians are creating now, sometimes adding themselves to the videos. https://www.404media.co/k-pop-fans-deepfakes-ai-generated-idols/

Artists deserve respect. They shouldn't be treated as puppets in fanfic, or have their work and characters treated as playthings and puppets by fans.

Mostly based on the film Brokeback Mountain. I even had a small, but dedicated, fanbase. I haven't written in anything in years, though.