Syncthing is a continuous file synchronization program. It synchronizes files between two or more computers.
It has a Web UI where you can manage the folders you want to sync and the remote devices.
Setting it up on my iPhone required using MobiusSync. And to be able to sync the vault folder into the Obsidian folder of the phone, I had to pay a one-time fee to sync “external” folders, i.e. not in the MobyusSync directory. All worth it, now my notes are synced up on both devices.