Why does iTunes on my new computer show an old library despite copying the correct files?

See Move your iTunes library to a new computer - Apple Community for general advice.

When iTunes is first installed it looks for a library file at C:\Users\<User>\Music\iTunes\iTunes Library.itl. If it isn't there it creates one. Double clicking an .itl file opens iTunes, but with the last library file it successfully opened, which may not be the same thing. Changing the media folder in iTunes does not cause it to scan the contents of the folder, it dictates where new download, rips, conversions, or consolidations end up. To switch from one library to another, or to create a new one, press and hold down Shift as you launch iTunes, then click Choose or Create as appropriate.

1) iTunes Music is the old name for the media folder. It became iTunes Media (at least for new libraries) back in iTunes 9. Using the old name can lead to issues, not least with some newer builds that in some circumstances reset the location to iTunes Media in the same folder as the .itl on every launch.

2) & 3) Uninstalling and reinstalling iTunes should have no impact on the current library, although installing an older build can trigger a launch time error where iTunes refuses to open a library created by a newer build. This error can also occur occasionally when the path to the active .itl is corrupt in some fashion. When you are signed into your Apple ID you should see your unhidden past purchases available to stream or download. If you have an active subscription to Apple Music and have enabled sync library/iCloud Music Library this is extended to your iCloud Library.

4) & 5) Have you checked the modification dates on the library file that you copied over from your old computer. A common issue that people run into at some point if their library doesn't work correctly it that they connect to another file in the Previous iTunes Libraries folder which then becomes the library that is updated every time they use iTunes. Unless you are consciously aware you have done this it can be easy to overlook when trying to migrate the library at a later point in time.

Device backups are not stored within the iTunes folder and are not normally migrated from one computer to another. It is possible, but generally not necessary. Simply backup the device on the new computer once you have resolved the issue with the library. See Archive iPhone backup in Windows - Apple Community for further background and Relocate iOS device backups for advice on changing where this backup data is stored, which can otherwise be an issue on newer computers with small system drives. The same user tip also has advice for storing your library on a different drive should that be needed.

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