Apple used to refer to a feature they called iTunes in the Cloud which shows all of your unhidden past purchases when you are signed into your Apple ID with cloud links to stream or download any which are not in your library.
iCloud Music Library is a feature of the iTunes Match and Apple Music subscription services. The first captures your ripped music that isn't from Apple and treats it in much the same way as iTunes in the Cloud, allowing you to stream or download items to other computers or devices that are signed into your Apple ID. The second adds in access to nearly everything in the iTunes Store, again letting you stream or download on demand.
iCloud Music Library has always required a subscription. Apple Music offers a three month free trial.
If you have content on your device that isn't in your library see Recover your iTunes library from your iPod or iOS device.
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