This addon is what I wanted. Thanks to the dev.
But this addon isn't perfect yet. First, the color of the interface being added does not match the background. Second, the placement of the interface makes the layout ugly.
I would like an option to hide the index number on all screens.
This review was generated by machine translation. I'm sorry if there is a part where the intention is difficult to convey.

Cette extension rajoute des fonctions supplémentaires sur YouTube:
● Sélectionnez la vidéo {0} à {1}
● Déplacez après ici (un ensemble de vidéo)

Please add an option to add things to a playlist (if you are in a playlist marking things)

Edit: I mean it would be great to select videos in one playlist and then be able to add them to a different one

Great for what it does. Missing a couple features I would like
1) Shift select/deselect to toggle a range. When on the liked videos playlist, I usually need to select a range of related recently watch videos and having to click each check is very time consuming.
2) Add to playlist. I use this for batch categorizing my liked songs. You can still do this by using add to queue button, and then youtube's own save queue to playlist feature.
3) De-duplication feature for playlist/queue, for anyone stupid enough to push save to playlist or add to queue twice. Youtube used to have a builtin de-dup button, but they got rid of it. Not really a feature I would expect from this, but one that wouldn't require much code change to do.