Restoring music in iTunes from MacBook to iPhone 14 Pro after iOS upgrade

Recently upgraded from 8plus to 14pro iOS 18.5. 8plus backed up to iCloud for transfer. Backup also on a 2009 MBP mid2009 OS 10.16.6 El Capitan.

Music in iTunes. Laptop backed

After transfer >50% of music was not there and ? gone from the 8plus. The 2009 MBP still has the files and has been backed up regularly to Time Machine.

Apple Store Expert said do an Airdrop from one laptop to other.

Didn’t work.

Online Apple support revealed that MacOS 10.11.6 was not compatible, but an older macOS only 1-2 updates older was.

Can the OS on the laptop be reverted to the older version through Time Machine and is it advisable?

[Re-Titled by Moderator]

Original Title: Restoration of music