PLEX with NAS newsbie

I have some basic questions that are probably very simple for many of you but I can't seem to find straightforward answers anywhere.

I simply want to watch my PLEX media on my AppleTV on demand, regardless of whether my MacBook is on or asleep.

I assume this means I need to store the server and the media on an NAS. However, how exactly does that work? Does the AppleTV stream directly from the NAS? I can't decipher how the media from the NAS gets to the AppleTV/iPhone if not via WiFi.

Thanks for any help