I’ve used Orgzly on Android. It is not as rich in features as GNU Emacs (it’s not going to invoke a python interpreter to evaluate embedded snippets, for instance), but for ordinary outlines / to-do lists and stuff it works fine. With Syncthing, the workflow is, I have a “sync” folder, and a “notes” folder inside it, and I just dump things in there until there are enough individual notes on a particular subject that it makes sense to stuff them into their own folder (but org-mode makes very large, deep outlines pretty manageable). On Android, the Syncthing app is able to do its job at the filesystem level, and you just need to open/save notes in that folder. You do also need to run Syncthing on a VPS or something if you want your notes to sync while you are not connected to your home WiFi or when your other devices are shut off (paying for a cheap VPS and maintaining it in lieu of paying for a specific service).

Org mode has support for linking to other notes, though I haven’t really taken advantage of that. In practice, most of my notes are not synchronized, and live in the same folder as whatever projects they pertain to. I once had a Syncthing instance running on the open internet (and it was pretty cool), but I don’t do this currently. Having Syncthing running was also useful for syncronizing my keepass password database, configuration files, and other useful doodads.