Syncthing is not a cloud storage or tool for sharing. It can be used like this on a stretch, but it’s a continuous two-way synchronization tool.

I portrait it like this: select a folder on one device, select a folder on second device, Syncthing would keep their content synced as if there were one folder :).
This is in contrast to Nextcloud that needs central location and user, to rsync that is oneshot and not two-way.