Obsidian offers a paid syncing service, and it’s probably well worth what they’re charging for it, but I had heard really good things about Syncthing so I decided to give it a shot.
Apple Ecosystem
One of the reasons I gravitated to Obsidian is that it is truly cross-platform. I am writing these words on a super cheap ThinkPad I got off of ebay, running the most excellent Zorin OS. If you are only using iPhones, iPads, and Macs, you don’t really need to do anything special, because Obsidian for iOS is going to automatically save your notes to iCloud, and when you open Obsidian on your Mac, you browse to iCloud / Obsidian and open one of your vaults.
Enter Syncthing
In fact, I am still using iCloud to sync between iOS and macOS, but I have other computers (and my work laptop is not allowed to have my Apple ID on it), and that’s where Syncthing comes in. I have an ARM server running on Oracle’s generous free tier. So I am syncing to that server from my Mac Mini, and from there I can sync to this laptop, my work laptop, my Windows machines, wherever I like.
Syncthing is easy to set up. You download a binary from their site and then you run it. The main difference from something like Dropbox or OneDrive is that it’s peer-to-peer. Every computer with Syncthing on it has its own admin dashboard, and if you want to sync a folder between two machines, you add the folder to Syncthing in both places.
From Linux to iOS
So as I write these words on the cheap Linux laptop, the words go to the Mac Mini via Syncthing, and then iCloud picks up the changes which will be visible on my phone without any other steps.