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For the old YouTube site I used to do it this way: https://webapps.stackexchange.com/a/70934/171425 . So I just could open someone's playlist and save it - add all videos to my new or existing playlist.
But I cannot figure out how to do it with newest YouTube site, is it even possible now?
New YouTube playlist page looks like this:
YouTube's new interface named "Polymer" disables the playlist duplicating/editing functionality of another channel's playlist.
A workaround (for the time being) is to temporarily disable the new interface. To do so, navigate to the desired playlist's page (as shown in the OP's screenshot) and add either "?disable_polymer=1" or "&disable_polymer=1" onto the end of the URL.
Example: https://www.youtube.com/playlist?list=PLda3VoSoc_TR7X7wfblBGiRz-bvhKpGkS&disable_polymer=1
After you enter the new page, click the three dots at the top-right corner, and add all of the videos to a new playlist. You can now go to your new playlist and edit it however you wish.
You can click on the the 3 dot menu and then click on RESTORE CLASSIC YOUTUBE. Then it should appear is it was the old and then you can repeat the process that you did before.
You may try to use diskyt.com.
Documentation how to copy from YT: Copy from YT Documentation how to copy to YT: Copy to YT
There is one catch. To allow copying back to YT you need to grant temporary access to your YT channel to diskyt.com. It may be safer to use some intermediate/throw away YT channel first if you are paranoid enough.
Disclaimer: I work on diskyt.com.
