Syncthing is an open-source and superior 1 Dropbox alternative that provides end-to-end and full-sync functionality on Windows, Mac, Linux and Android (but not iPhone).
On Android 2 you can use the Obsidian app to edit your notes synced by Syncthing. Changes made on desktop and mobile propagate both ways automatically.
Dealing with data loss
Data loss is not uncommon when working with syncthing. For this reason, it is recommended to also track your notebook in a Git repository. Assuming you commit often (via Visual Studio Code extensions like this), if any file goes missing you can always recover it from Git.
Git also acts as backup if you push your repository to a remote location (GitHub or a private server via SSH).