The absence of fundamental music app functionality in Google’s current streaming service is a major complaint for some users. One such omission is multi-select in list views, although YouTube Music is currently introducing that feature.
Update 7/2: Today, multi-select is being rolled out more broadly on the YouTube Musics web app. Hovering over a song when browsing an album or playlist causes the track duration to be replaced to the right by a checkbox with thumbs up/down and the overflow menu to appear to the left.
After choosing one song, the box for every song in the list displays, and you also get a bottom toolbar with quick Add to playlist and Play next buttons. Add to playlist, Play next, and Add to queue are available on the overflow menu.
Original 6/1: Some users are observing that music tracks.
A checkbox is now visible in list views on YouTube (to the right of thumbs up/down and the overflow menu). This likely shows when you hover over a song and is still visible after you make at least one choice.
At the bottom of your screen, YouTube Music will display a box with choices for adding to playlists and queues. Another overflow menu is also available here.
Multi-select is currently only available to a small number of online users. The ability may take more development work than the web client does, thus it is unclear if it will also be available on the Android and iOS clients. In the mobile apps, the overflow menu can already be opened by long-pressing a song.
However, the fact that this capability is available anywhere is progress, and the addition offers users at least one location to perform bulk activities on their library. Hopefully, YouTube Music will soon roll out multi-select to more users.