My guess would be that each of their devices (phone, laptop, etc) syncs back to their server/NAS, but they do not sync to each other. The server/NAS is the hub, and each device is a spoke.

Yeah I saw a post about it a long time ago on Reddit for users with lots of devices

Basically it is just setting up one or two “central devices” that know all the client devices, but not linking the client devices individually.

IE: One server is connected to your phone, laptop, tablet, desktop, etc. But the phone is not directly connected to your laptop or desktop or tablet.

To be fair I don’t actually know if this is the best approach anymore or if just connecting all of them in a mesh is better 🤷

Here is a forum post describing it.