If you want to copy a Youtube playlist that someone else has made, you can't do it normally on youtube. But if you go to the same playlist on music.youtube.com you can copy it.

Here's a playlist you can test this with. If you go to the first link you'll see three dots next to the "play all" button. Click that and you'll see a few options.

Now do the same with the second link and you'll see quite a few more options including "save to playlist". That's what you use. Bob's your uncle. With this you can combine various other playlists into one, then you can edit the playlist to your liking.

https://www.youtube.com/playlist?list=PLCZIvsaECVuIeehs9pP4IgmVcQNXh07WI

https://music.youtube.com/playlist?list=PLCZIvsaECVuIeehs9pP4IgmVcQNXh07WI