Plex, PiHole, Photoprism, Home Assistant, Syncthing in a hub and spoke config, Caddy for reverse proxy, custom containers for: yt-dlp, restic, and rsync.
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.