Queue editing is perhaps one of the best features you can find on music streaming apps. If you're jamming to a playlist, but you want to mix things up with a song that's not in that playlist, or farther back, you can just look up the title in question, add it to the queue, and sit back and enjoy it when it's played automatically. YouTube has a function just like that, but for videos, on the web. You'll soon be able to create a queue on your smartphone as well, as YouTube is porting this feature to Android and iOS devices.
YouTube has added a button to add videos to your queue, and it can now be accessed by YouTube Premium subscribers through the app's "Try new features" section. Once you turn it on, the three-dot menu on videos will now show a new "Play last in queue" button that will automatically add that video to your playing queue. If you're letting auto play run, you'll see that video you just added right after whatever many videos you currently have in the queue. If you close the video and the mini-player, the queue will clear.
You can add a handful of videos and make a temporary playlist that won't be saved after you're done with it. It'll definitely come in handy if you frequently watch multiple videos in a row while using YouTube. As of the moment, the feature is only live for Premium testers. It'll be available until January 28th, at which point it will presumably either be removed or moved into production. If you want to give it a spin, make sure you're subscribed to YouTube Premium to get the new testing panel to show up—and if you're not, keep in mind it recently went up in price.