What triggers Apple Classical app to download a copy of music locally?

The User Guide for Apple Classical says : > You can use the Apple Music app to download albums, playlists, or tracks that you added to your library But I am finding copies of albums appearing on my iPad in the Settings app, on General > iPad Storage > Music panel. I have only used the Apple Classical app, not the Apple Music app. So what is triggering this local caching? Is it when I press the star icon or choose to “favorite” an album? And how is marking a favorite different than “Add to Library”? And what is the Library, does that mean keep a copy locally? Is there a way to just remember an album so I can return to it later without making a download copy?