Came across an album that is not on Spotify thus difficult to stream to my Sonos at home. Thinking about how to get that and other old MP3s to stream and thinking a NAS.

Tell me about NAS like I'm a 5 year old, with just enough info for a somewhat technical but not super technical person to understand.

Seems like you buy the NAS box then separately the HDDs to install?

I have ancient HDDs backed up to Dropbox and Apple products to their clouds. Can I, and should I, drag those files to the NAS and then backup from NAS to those clouds?

Several hundred $ to listen to the random MP3 won't fly with the pocketbook so sell me on why I need one beyond old MP3s?

Why don't you just play it from your phone to the Sonos?

Just save the album/songs on your phone and play out of Spotify. Spotify can play songs from your local "library"

I wouldn't deal with a NAS unless you had 100's of GB's worth of music you needed to save and stream from. I only use my NAS as Plex server for watching my movies and TV shows that I've ripped from the physical platforms.