Syncthing for me https://github.com/Catfriend1/syncthing-android
ObsidianMD
Unofficial Lemmy community for https://obsidian.md/
Exporting the .md file. I also think that for fellow Obsidian users, the bits that have functionality in Obsidian that don't in Markdown in general might come through if they open the file in Obsidian, though I'm not 100% sure.
I know there is probably a plugin for this, as well as the official Obsidian Sync.
https://help.obsidian.md/getting-started/sync-your-notes-across-devices
Out of the options listed here, Syncthing seems to have compatibility with all of your things.
Put them in a git repo
I went with official Obsidian sync which also helps the dev to keep developing Obsidian.
One good thing this sync everything including keepass vault. So I kind of use it as file sync to keep keepass sync across Mac & Linux.