Your entertainment centre has to have a modern receiver that's integrated into your home network via LAN or WLAN. Then you just have to tell your receiver the NAS IP address so it recognizes it as a media server.
No, because a network hard drive is integrated in the FRITZ!Box whose capacity can be expanded almost as you wish via USB. It can also be used on-the-go via the internet and setup requires just a few mouse clicks. However, a FRITZ!Box doesn't have as much computing power and it doesn't offer as many convenience functions like auto backup or photo and media galleries.
You can use the NAS manufacturer's app to access your network hard drive whether it be Apple or android. If you have a PC, MAC or laptop, you have two options: either access your network hard drive via your browser by entering the NAS IP address in the address bar or integrate the network hard drive as a network drive. To do so, use the "Finder" on a MAC and "Explorer" on Windows.