The YouTube desktop already has the ‘Add To Queue' feature that lets you add videos that you would like to watch next after you are done watching the current one. When you use the ‘Add To Queue' feature, it gets added to a list below your mini-player. Now, YouTube is testing the new feature for Android and iOS.
Currently, on Android and iOS, you can use the ‘Save To Watch later' or ‘Save To playlist' features to get back to the videos that you would like to watch at a later time. However, using these features saves the video permanently, and you need to manually delete them after you have watched them. Adding a video to a queue is temporary, negating the need to manually remove it later. Once the feature goes live for Android and iOS, you can tap on the 3-dot overflow menu next to the videos and select the ‘Play Last In Queue' option at the very top of the list. Do note that this won't apply to YouTube Shorts.
After you have added the video using the above option, it will be added to a queue at the bottom of your mini-player. You can easily drag the videos to rearrange them in the queue. Also, you can remove the videos in the queue by swiping left. There are other options to repeat and shuffle, as well. Closing the mini-player also clears the queue.
If you play something, it will be added to the queue and can be moved back. As noted by 9To5Google, the ‘Line Up What To Watch' is notably advertised as a YouTube Premium feature and is available for premium subscribers. In order to use this feature, you can tap on your Profile Avatar, select Settings, and hit Try New Features. Once selected, it may take a few minutes to load the feature. The feature will be tested on YouTube for Android and iOS until January 28 and will soon roll out after that.