The key is to enable iCloud Music Library under Edit > Preferences > General in iTunes. That then enables the "Add to iCloud Music Library" right click option over albums and individual files. Items you "add to iCloud Music Library" will then automatically appear in the Music app on your iOS devices.
Syncing the music to your iPhone or iPad
You can also choose to add songs to iCloud Music Library from PC manually. In iTunes: Choose the songs you want to add to iCloud Music Library > Then right-click > Choose Add to iCloud Music Library option. That's all. Now you can access your songs on any device when you are connected to the Internet.
On Mac:
iCloud Music Library allows PC, Mac and iOS users to store their personal music library online, by matching uploaded tracks to songs listed on the iTunes Store or uploading tracks directly if no match is available.
There is a number of plans you can subscribe to from a free 5 GB (just to try maybe) or a family plan (up to 1 TB, $9.99 per month). From this point on, all the music that you buy on iTunes will be available in your iCloud Music Library.
To turn on iCloud Music Library:
Play, download, or remove songs from iCloud Music Library on PC
Reset Your iCloud Music Library | Tom's Guide.
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Part 2: How to Add Songs to iCloud Music Library on Mac/PC Step 1. To begin it, run the latest version of iTunes on your computer and then select the song you want to add to iCloud. Step 2. Next, click on 3 dots next to the song and then, finally, tap on “Add To My Music” option.
Click on the Music app on your Mac. Choose + Add or + next to the song, album, or playlist you wish to add to the iCloud Music Library. Select Recently Added under Library on the left side of the app, then confirm the addition of the content to the library.
Run iTunes > Click File option > Choose Add File to Library from the drop-down list > Choose the songs you want to add to iTunes Library. In iTunes: Click Edit > Choose Preferences > Click General > Check the box next to iCloud Music Library to enable it > Click OK to confirm.
Your original library is scanned into iCloud, but your songs are yours, and Apple will not automatically delete them, or replace them with its own proprietary copies.