Google's move from Play Music to YouTube Music is still unresolved – especially since the final transition of libraries and uploaded content between the two still has not happened – but the latter is picking up a new feature to help with offline playback. "Smart Downloads," as it's called, wants to pull down up to 500 songs that they've "liked" via that button.
The new Smart Downloads builds upon the popular offline mixtape feature, and it does not seem to be right now, though 9to5Google spotted it in a teardown all the way back in January, and there's some indication that it was in limited testing later. Precise details therefore do not seem to have been formally announced, with only The Verge and Engadget reporting on this.
Smart Downloads offers some configuration options (like limits for the volume of music to pull down and store) concern, and downloads are set to only happen at night.
Google gave The Verge a description, "The Mixtape", Pop Hotlist, Summer 201
Again, youll need a YouTube Music Premium subscriber (or one of the older, bundled, grandfathered YouTube / Play Music plans) to get that new feature, so keep that in mind. It's not clear when this feature is out, but it should be announced.