In September 2016, a server side change from YouTube has enabled a new bottom navigation bar UI for some users across the globe. However, the company quickly pulled back the feature and rolled back the old top navigation bar user interface.
Today, YouTube officially said that all the users across the globe will get the new YouTube's bottom navigation bar UI. YouTube is making this change because customers can reach out all the buttons with their thumb.
The bottom bar has all the same tabs as the top navigation bar, but the account tab has been replaced with a new library tab. Also, the application is becoming smarter as it resumes from where you have left off.
YouTube says that the feature is now rolling out for the users of iOS, but Android users have to wait a little more to enjoy the bottom navigation bar UI.