YouTube Music update adds new Smart Downloads feature, here is how it works2
Engadget reports that new feature will be available on Android devices first, but only for YouTube Music Premium subscribers. In order for the feature to work, you'll have to enable Smart Downloads from the app's menu. All downloads will happen while your phone is connected to Wi-Fi, hence the name Smart Downloads.
Make sure that you have enough free storage though and if you don't think your phone can take all your “liked” songs, you can select how many you want YouTube Music to download from within the settings.
Unlike Offline Mixtape, a feature that's available for YouTube Music users for quite some time, Smart Downloads doesn't create a mix of songs that fit your taste profile and makes it available offline but rather downloads exactly the songs you like.