Syncthing

Syncthing synchronizes folders directly between devices with no central server: every machine holds its own copy and changes propagate peer-to-peer over an encrypted protocol. It has quietly become one of the most reliable pieces of software in the self-hosted world.

There is no web-accessible "cloud" view of your files and no sharing links — it is sync, not storage. Conflict handling is sensible but worth understanding before you point it at anything precious, and remember: sync is not backup.

The default answer for keeping the same folders identical on a laptop, desktop, phone and NAS.