I use Syncthing, a cloud-free continuous file synchronizer, since ever so many years (syncing ~1.3 mio files currently, between 13 devices) so I can work on my desktop at home and on the laptop when I’m away. It also syncs with my Android smartphone.
It syncs between my laptop and desktop Obsidian vaults. The Android phone is just for quick notes, I sometimes even dictate into it, so it has no Obsidian (yet?) but just syncs into an “Inbox” folder within my Obsidian vault.
So I can take quick notes on the road, and later edit/move them within Obsidian on my laptop or desktop.
Be aware that on Android, SD card file systems often get initialized using some FAT file system which might cause problems when “the other end” is MacOS or Linux. The safest bet is to use the phone’s “Internal storage” for syncing with Obsidian.
Oh, and Linux/Android file systems are mainly case-sensitive, while Windows and MacOS (I think?) are usually not. This might turn out to be a caveat.
There is, unfortunately, not yet an official Syncthing client for iOS devices.