Yeah, I'm using Scrivener. Before that, yWriter, basically a clone with less features, but free. Loving them both, even though I'm only using Scrivener these days.
Writing
A place to share ideas and resources about writing.
I use Manuskript, which is a pretty nice FOSS writing program. It lets you define a whole encyclopedia about all aspects of your story (characters, locations, magic, technology, etc.) and breaks the whole thing up into scenes that can be reordered at will, put into folders to create larger sections (chapters, or even sub-chapters), etc. And, when you're done, you can have it export the project (any/all of it as you wish) to just about any format: text, Word, PDF, whatever.
I've also been toying around a bit with Obsidian. It's not really meant as a writing app, but you can definitely do deep outlines as well as all of the same type of encyclopedia/wiki setup to help you keep track of everything.
To be honest, I just stick to plain ol' LibreOffice. I use folders to organize notes, which isn't perfect but works for me.