Others have mentioned Syncthing as a sync solution. I’d like to add a couple points:

Syncthing can work fine even for solutions that are intended to use their own sync, provided it’s a single-user setup. You’re not likely to make simultaneous changes on 2 devices, so collisions are unlikely.

Also for using Syncthing, I recommend Syncthing-Fork for Android - it moves sync conditions into the folder/job rather than global. Very useful when you have jobs you want always syncing, and jobs you want to only sync on wifi and power.

If using iOS there’s an ST client called Möbius Sync ($5), developed by a company the financially supports Syncthing.

For Windows, get SyncTrayzor - it makes running and managing ST easier.