Missing Music on new iPhone

I had an iPhone XR, was trading it in for a iPhone 15. I had to upgrade to iOS18, so I did that and backed up my phone to iTunes. Shortly thereafter, my new phone arrived and I synced the two side-by-side. I then reset my old phone and shipped it off for my trade-in.

Skip forward a week or so ... and I found that all my "old" music was missing from my new phone! By old, I mean the music that I added, not what was purchased. I checked iTunes and the music was also missing from there. Since I had listened to various of my "old" songs not long before all this transpired, I deduce that something with the original iOS18 deleted my music.

Where and how can I get my music back??

[Re-Titled by Moderator]