By default, syncthing uses public relays, but thankfully that can be disabled

It's important to say that data cannot be decrypted by the relay

One reason to use your own relay/discosrv (if anyone wants one) is that some of the public relays/discosrvs are also hosting Tor nodes, so Ubiquity's routers will tell their Admin you're using Tor.

https://docs.syncthing.net/users/relaying.html + https://docs.syncthing.net/users/strelaysrv.html