Emacs is seriously the best 'app' for anyone working with text, including writers. And if its (default) looks put you off, have a look at extensions (and Emacs is designed to be extended) like the Poet theme: https://github.com/kunalb/poet
Amen to that. I read a blog post somewhere (sorry, lost the reference) about a writer (non-programmer, mind you) who switched to Emacs for Org-Mode.
Another extension that's great for writing/note taking is Deft [1]. Give it a directory (optional recursing) of .md, .txt, .org, etc. files, and it will let you filter through them instantly. I've been using this for a few years now and it's still my primary digital note-taking system.
Another great Emacs extension for writing is writeroom-mode: https://github.com/joostkremers/writeroom-mode