Some of my songs are missing after syncing to my iPhone

My wife recently upgraded from an iPhone 8 to a new iPhone 15. We synced everything to the icloud and then to the new phone while at the Apple Store, but didn’t realize until later that not all songs made it to the iTunes app on the new phone. There doesn’t seem to be any connection between songs that transferred and those that didn’t as some were purchased from iTunes and some were transferred from CDs years ago.

[Re-Titled by Moderator]