restored iPhone is not the same

Backups do not include the iOS version, only data from apps and settings. So if you restore a backup that will not revert the iOS version.

What specifically did not restore? Here are a few things that are not included in a backup:

1. Any data that you sync (not back up) to iCloud, such as photos
2. Contacts, Notes, Calendar events and Reminders, because they sync to your email server(s), and will be restored when you log back in to your email account(s)
3. Stolen Device Protection settings (so you can bypass SDP if your biometric identification is broken)

Beyond these, everything on your iPhone will be restored to what it was at the time of the backup.

Downgrading your iOS isn't possible. What you were trying to do of restore from a previous backup only works when you are on Beta software and the backup is on public stable software.

To the best of my knowledge, music from your own CDs also is not restored by a "restore".

if you had any of that which you had manually synchronized from a Mac or Windows PC, you would need to do a new manual synchronization to get the music back onto the iPhone after the reset.

I feel sure this was possible and I've done it previously.

Possible that you are not remembering all the details?

If you have installed a beta version of the iOS.....for example you installed a beta version of iOS 26.......it is possible to revert back to iOS 18......during the beta period.

But....if you installed the official release version of iOS 26.....you cannot revert to iOS 18 using any method that Apple supports, no matter how long ago you are talking about. The same is true regarding any other iOS that you might have used in the past.

So, you are either thinking about a previous time when you reverted the iOS during a beta period......or.....if you really did revert after an iOS version was officially released, you jailbroke your phone to do this.

If any of the veteran experts on this site see this post, they will confirm that what I have said is true.

If you were trying to revert to the previous version of iOS, you can't do that. Downgrading is not supported.

All bets are off if you are telling us that you installed the release version of iOS 26 and you tried to roll back to iOS 18. This is not supported by Apple.

Is that what you did?

It is sometimes technically possible for a short time. It is not, however, supported, and we can't help you do it.