Very recently, I worked on a couple of iMacs working to move things from one to another. I was able to do some backups and get them set up with iCloud. I have gone through their extensive Music folder and cleaned it up. There are over 12,000 songs in the main library (plus ~40,000 in a Jazz library).
The question I'm being asked now, is how they can play this music on their iPhone - without paying for Apple Music.
iPhone 12
Hello Letter17,
We understand that you'd like to transfer music from the iMac to an iPhone. Is that correct? If so, the following steps will be able to assist you:
"Sync music to your device
You can connect your device using a USB or USB-C cable or using a Wi-Fi connection. See Sync content between your Mac and iPhone, iPad, or iPod touch over Wi-Fi.
2. In the Finder 
on your Mac, select the device in the Finder sidebar.
If you connect your device to your Mac using a USB cable and don’t see the device in the Finder sidebar, see If your device doesn’t appear in the sidebar.
3. Click Music in the button bar.
4. Select the “Sync music onto [device name]” checkbox to turn on syncing of your music.
With the checkbox selected, syncing is set to transfer all your music to your device.
5. To sync a selection of music, click “Selected playlist, artists, albums, and genres.”
6. Select the checkbox for each item you want to sync in the music list. Deselect the checkbox for any item you don’t want to sync.
Click Artists, Albums, Genres, or Playlists to see your content organized by that category.
7. Select syncing options:
8. When you’re ready to sync, click Apply.
You can choose to sync your Mac and your device automatically whenever you connect them. See Turn automatic syncing on or off on Mac.
Before disconnecting your device from your Mac, click the Eject button 
in the Finder sidebar."
More details here: Sync music between your Mac and iPhone, iPad, or iPod touch
Best wishes!
Hello Letter17,
We understand that you'd like to transfer music from the iMac to an iPhone. Is that correct? If so, the following steps will be able to assist you:
"Sync music to your device
You can connect your device using a USB or USB-C cable or using a Wi-Fi connection. See Sync content between your Mac and iPhone, iPad, or iPod touch over Wi-Fi.
2. In the Finder on your Mac, select the device in the Finder sidebar.
If you connect your device to your Mac using a USB cable and don’t see the device in the Finder sidebar, see If your device doesn’t appear in the sidebar.
3. Click Music in the button bar.
4. Select the “Sync music onto [device name]” checkbox to turn on syncing of your music.
With the checkbox selected, syncing is set to transfer all your music to your device.
5. To sync a selection of music, click “Selected playlist, artists, albums, and genres.”
6. Select the checkbox for each item you want to sync in the music list. Deselect the checkbox for any item you don’t want to sync.
Click Artists, Albums, Genres, or Playlists to see your content organized by that category.
7. Select syncing options:
8. When you’re ready to sync, click Apply.
You can choose to sync your Mac and your device automatically whenever you connect them. See Turn automatic syncing on or off on Mac.
Before disconnecting your device from your Mac, click the Eject button in the Finder sidebar."
More details here: Sync music between your Mac and iPhone, iPad, or iPod touch
Best wishes!
Hello letter17,
That is correct. If you moved the music files into iCloud Drive, you would need to move them back to the computer in order to play or sync them.
Download items stored only in iCloud Drive to your Mac
In some cases, items may be stored in iCloud Drive but may not yet be downloaded to your Mac.
1. Control-click the items that you want to download (indicated by the In iCloud status icon).
2. Choose Download Now.
Work with folders and files in iCloud Drive
Cheers!
To iterate, all of his music is in iCloud. We took an old iMac and selectively did a backup. I then went through all of those files and cleaned them up. The music is not in the music folder on his new iMac, just in iCloud. Does he have to download everything to his iMac to the music folder to be able to sync? The 40,000+ track Jazz Library is also separate so not to be so cumbersome. That that also have to be moved to the music folder and downloaded to his iMac music folder?
That's unfortunate. The whole point of having things in iCloud is so they don't have to be on the computer.
iCloud Music
