The queueing system is being tested on the two mobile operating systems. The feature has been available on the web for years now, and shows in the YouTube apps under certain circumstances, as will those who use the YouTube Music apps.
You will be able to play last in queue after you turn on the feature, which we will cover in just a second. You can either add the video to the bottom of your queue or create a new queue if you aren't watching a video at the moment. The app will play the next video in the queue until you run out of time. The videos can be rearranged or removed from the queue. The queue will be deleted if you close the player or tap the "x" button in the bottom bar.
When I opened the app on December 24th, I was greeted with a screen that said the feature was available to test, and a button to turn it on. The feature was rolled out by YouTube earlier this month, but the pop-up didn't show up for me until this morning. If you didn't get that screen and you're a Premium subscriber, you can manually enable it by scrolling to "Try it out" and tapping the "Try it out" button.
The test will be available until January 28th.
The test is not a sign that non-paying users will be able to queue up videos in the future. The app failed to add a video to the queue at one point, apparently because I tried to add another one too soon. I hope I will be able to rely on this feature in the app someday, as it is something I use almost daily on the desktop.
There's a queueing system in the YouTube Music app.