When you watch a video in fullscreen mode, the idea is that when you’re in fullscreen, it creates a more immersive experience where you aren’t distracted by other parts of the UI and such. At the same time, this can also limit the things you can do while in fullscreen, especially on platforms such as YouTube.
That appears to be changing because according to 9to5Google, YouTube has recently pushed out an update to its Android app where they are giving it better fullscreen functionality. As you can see in the screenshots, instead of just giving buttons that let you pause/play the video or fast forward or rewind, the additional buttons in fullscreen mode will let you do more.
This includes liking or disliking a video, pulling up comments, adding it to your playlist, or sharing it. Prior to this, those functions were only available if you were to exit fullscreen mode. As you might know, many creators tend to ask their viewers to like, share, and subscribe to their videos, so having the ability to do that while watching the video could encourage more users to do just that.
These changes are reportedly being pushed out to users as we speak, so if you don’t see it yet, don’t worry as it should eventually make its way to you. This only seems to be applicable for the Android app as iOS users are still on the previous UI design.