I have used many open source notes taking apps. My goto used to be Zim Desktop Wiki [0] but its just a desktop app and the was no built in sync solution. On mobile I used Markor [1] which understood Zims syntax, as well as markdown.
Due to lack of mobile client and built in sync options I moved to Joplin [2]. Its markdown, cross platform, and i can sync with WebDav. People don't like that its SQLite based, but honestly worse com to worse your notes are an export away, or you can just open the database yourself. Im happy with it.
I always recommend Treesheets [3] though, excellent for brainstorming/rapid note taking.
Due to lack of mobile client and built in sync options I moved to Joplin [2]. Its markdown, cross platform, and i can sync with WebDav. People don't like that its SQLite based, but honestly worse com to worse your notes are an export away, or you can just open the database yourself. Im happy with it.
I always recommend Treesheets [3] though, excellent for brainstorming/rapid note taking.
[0] https://zim-wiki.org/
[1] https://f-droid.org/packages/net.gsantner.markor/
[2] https://joplinapp.org/
[3] https://strlen.com/treesheets/