On mobile (Android) I use 'Markor' (it's on F-Droid), which is a notetaking application for Markdown, that understands folder hierarchies as notebooks and thus

On mobile (Android) I use 'Markor' (it's on F-Droid), which is a notetaking application for Markdown, that understands folder hierarchies as notebooks and thus is compatible with Obsidian on the desktop. I use Syncthing to sync.