A backup only backs up data, not the media. Media is music, videos etc.
If you have music on your phone that you purchased from the iTunes Store using the phone, and which is therefore not in your iTunes Library on your computer, the method is to sign onto your Apple account in your iTunes Library and use iTunes itself to download your purchases from the store to your Library. To back up that music, you need to use your computer's file manager, not iTunes itself.
If you have music on your phone that came from other sources, such as Amazon or Bandcamp for example, then go to that store on your computer and download those purchases from your account with that store. They then become the backup for the copy on your phone and you can decide whether to add those songs to your iTunes Library as well if you wish (logically, you would). However, in both cases (the iTunes store or other sources) you should backup the music to another drive because the phone is not a backup for what is on your computer.
dave—- wrote:
<...>
Not clear as to iphone will over write itunes or other way round or mash together
If you use Sync to manage your phone (from iTunes);
- that Sync will only update songs in your library with any changes made on the iPod to those same songs. For example, if you change a song's rating, the Sync will update the rating in the library. The sync also updates the Last Played time
- if your phone has been synchronised with another iTunes library, then attempting to synchronise it with this new library will cause a warning message informing you that media on the phone will be deleted and replaced only by what is in this new library. A "new library" not only means a different library on a different computer, but can also mean the same library after the computer has been repaired (and re-built)
If your existing computer has been re-built, and the iTunes Library is the same as it was before the re-build, then a Sync will erase the existing media but then put back the same songs because they are in the "new library".