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

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

go

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

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

@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.

Xournal++ is the most similar in intention I believe.

I personally use a combination of logseq (FOSS) and obsidian (not FOSS, freemium I think). Obsidian is currently better on the tablet due to some particularly well put together plugins and a recent feature update but I’d like to move completely to logseq long term. However, if you’re looking for a touch centric experience it’s hard to deny that obsidian is the best in the ecosystem.