It’s not. It’s just a GUI for a server.

It’s a modular GUI, though. The base install allows basic server management: system stats, upgrades, and a couple other bits. However, you can install additional modules to expand the GUI to allow for storage management, ZFS management, container management, file sharing (NFS, SMB), a file browser, VM management, user management, and so on.

My go-to “NAS system” is just standard Debian with Cockpit and a handful of supporting modules. It gives a nice GUI like OMV, but with all the flexibility and control of a normal Debian install.