Try Syncthing (Synctrayzor is a nice wrapper on Win, Syncthing-Fork on Android is better than original Syncthing), if you don't want to pay for their Obsidian Sync.
As long as you can sync a folder on your phone (whether with Git or any cloud service), mobile app can open it.
Of course, you may experience some conflicts, but that's why you can pay them not to worry about that. Totally worth the price for me, though I do have a permanent discount ($5/month, as does everyone that signed up before Sep 30th).
Edit: It looks like it's backed by git so maybe it could be used for free, with a bit of faffing. Unfortunately I'm cheap and lazy.