I use Obsidian on Desktop. They also have a mobile option. Could be worth checking out. https://obsidian.md/

jtxBoard

https://github.com/FossifyOrg/Notes

I’ve been using google keep. But I guess there are better ones out there.

Do. You. Want. Them. To. Be. Transmitted. By. Relays?

N. & O. S. T. R.

I use Joplin. It is open source and the notes are encrypted. It syncs with my other devices over a free DropBox account. You can create multiple notebooks, and even nest them inside parent notebooks. The app also supports to-do lists and tagging of notes.