A NAS is usually a bit more than just storage these days. It’s a home server.
However those with multiple bays will allow you to configure them in various ways of RAID, and exposes the whole lot as a single volume by default. Stuff like Synology will give you shares over the network to it that by default will be stuff like Music, Videos, etc. But you can configure it how you like and create whatever shares you like.
Synology have fancy things for cloud-like storage to sync from devices to the NAS (it’s in the “cloud” but it isn’t, it’s a private cloud on your NAS, so if that dies, it’s gone. Unless you back it up 😉 ).
Or you can use standard backup software on PC/Mac/whatever to a share on the NAS.
p.s. with RAID you have various options. Just bundle everything as one big store, stripe it for speed, and/or have a disc or two as redundancy in case one dies (less storage space but can cope with disks dying without losing everything).