How do I get my full music library back into my new iPhone 17 with iOS 26.0.1?

For years, the music on my Apple computers appeared automatically [?] on my iPhone. Most of this music is from CDs, personal recordings, etc. Only a small portion was downloaded from Apple Music. When I bought a new computer, I would simply copy the library and transfer it to the new computer

Now that I have a new iPhone and a new iOS, it looks like only the handful of downloaded songs is playable on my phone. The rest is greyed out. When I try to play a song, I get "This item is not currently available in your country or region."