Is the NAS (Network Attached Storage) the best external hard drive option for a home?

I have been looking at the WD MyCloud 4Tb version specifically to store my iTunes music library and any videos I may have in the future.

Been wondering if via the NAS, one can access their iTunes music in their cars without having to store the music on a local flash storage drive like on their iPhones. Everything can be remotely accessed through their NAS at home.

That is possible. By default on most home routers, however, there's going to be a firewall preventing anyone outside the LAN from accessing anything within the home. In order to do this, you would simply need to figure out the network details of the device (internal IP and port number), which you would then put into your router's port-forwarding settings. I'm assuming here that the device does not already have an inbuilt UPnP function, which would set up port forwarding automatically. Once you have that all set up, you should be able to access it anywhere, so long as you have an internet connection.