I really like Trilium. It’s basically a FOSS Obsidian. It does not have a phone app but the web interface is mobile-friendly.

I’ll throw out there that Emacs org-mode is an incredibly robust solution for this, although it may be a bit overkill for your needs (and may send you down an Emacs rabbit hole).

At least for me, it’s gotten to the point that my entire life can be summarized in a giant pile of *.org files. I highly recommend it

@NightOwl As already said, not foss, but obsidian is a really good option. I moved away from Onenot and Notion, and all my files are local in markdown.

Standard Notes is pretty great

go

I use Obsidian. It is simple and easy to use but very feature filled. It uses Markdown and stores files locally.

I personally enjoy Joplin as my FOSS note-taking app.