Music (I call it Music, not Apple Music, as Apple Music is an online streaming service that has the same name and confuses everyone) is iTunes under a different name. It works pretty much the same. Normally the library would just transfer over and there would be no issues.
Did you look for an iTunes folder under your user folder in Finder? You may be able to copy the iTunes folder contents (everything inside the iTunes folder, not the iTunes folder itself) to the Music folder and then open Music, and it may appear in Music, complete with tracks and playlists.
If you don't find an iTunes folder (usually caused by having it at the top level of the drive instead of under the user folder), you would recover a backup of your iTunes folder which would get you all your music and your playlists like they were before the upgrade. It is surprising how many people do not backup files. So of course there are ways to attempt recovery.
It is a good thing the iPod refused to sync with iTunes as it would have deleted the Music from the iPod, as it is not allowed to sync from iPods to iTunes/Music.
You can buy recovery software, to try and recover the deleted folders from the upgrade. This could work, but I wouldn't bother as it could get you nothing, or only a partial restore.
You can move the songs from the iPod to Apple Music.
- The first step is to prevent syncing automatically. Hold Option and Shift keys and connect the iPod to the Mac to stop a possible sync. Then open the iPod in Finder and uncheck Automatically sync when this iPod is connected.
- Create a folder on your desktop for the Recovered files.
- Next is to open the iPod in Finder. Double click it on the desktop, or select the iPod's name in Finder.
- Navigate to the iPod Control folder, then the Music folder.
- Copy everything in the Music folder of the iPod and put it in the Recovered files folder on the desktop. This may take a long time.
- After the copy is complete, then you open Music
- Open the preferences, and go to the Advanced tab. Check Keep Music folder organized and Copy files to Music folder when adding to library and click OK.
- Then use the Files menus, select Import, then at the top select your user folder, then select the Desktop folder, then select Restored Files folder, then click Open.
- This will put the tracks into your library based on the data inside the file (Artist, Album, Song) and organize it by Artist/Album/trackname.ext
- If any tracks were MP3 with incomplete or missing information, you may get some Unknown Artist, Unknown Album, etc. You will need to listen to these tracks to determine what that data is and you can add it by editing the Song Info for the track you listened to.
- Once the import is complete from the first iPod, you can do the same (starting at "The first step") with the second iPod, though you may end up with duplicate music, but you can delete those from the library as you find them.
Now you need to renable sync with both iPods by connecting each iPod to the Mac, then open the iPod in Finder and check Automatically sync when this iPod is connected. Warning:
This will allow the Music program to erase the iPod and sync from your library.
The Playlists might be recoverable with 3rd party software that you can buy. Some may change smart playlists to standard playlists (static) and you would need to recreate the smart playlists manually. I don't know what software would work. You may need to research based on reviews what would work for you.
Once everything is recovered, you can delete the Recovered music folder(s). Then I would make a backup of the Music folder to prevent the need to recover again.