NAS is 'Network Allocated Storage', basically a hard drive with a network controller attached. It allows you to share the contents of storage (HDD) on a network.
I don't fully understand what you're asking(!), but:
NAS hard drives are just hard drives designed to be used in a NAS. Typically they are quiet and use less power than normal hard drives. Yes they can be used in X20.
The X20 can be used as a NAS by setting up sharing in OpenWRT. OpenWRT is like another operating system, running simultaneously to Android on X20. You can use it to set up Samba (Windows networking), FTP, DLNA shares from the internal HDD of an X20 (or external storage and also on X20 Pro, Z9S, Z10). This guide is for X9S, but the key steps are the same for X20/X20 Pro/Z9S/Z10: https://www.zidoo.tv/Support/guide_step/guide_target/XGpkIb0CIARKKmVViAFMcQ%3D%3D.html