The update (left) and the unpublished version (right) on YouTube 13.01.4.Photo source: OustedHoChiMinh / Reddit, Jake Peterson / Gadget Hacks
Note : If you are using Apple's built-in "dark mode" when enabling YouTube, it will turn the user interface into white. You will have to disable "dark mode" on iOS first if you want to use YouTube. Just remember to re-enable iOS's "dark mode" when exiting YouTube or the rest of the iPhone will return to normal.
If you want to be the first to test the new features of YouTube on iOS, try signing up for YouTube for iOS beta (https://productforums.google.com). There is no guarantee you can join, because the beta version may be full, but at least you will be put on a waiting list.
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Having fun!