Merge two playlists created on YouTube

Hello, over the years I have created several playlists from YouTube. I'll keep it simple: I automatically save to "Watch Later." Ideal, just one click! But now I would like to merge my playlists and there, I admit that I am really stuck! Also, do you know the maximum limits of a playlist (I stop at 80 songs, just guessing!) A big thank you in advance to the kind soul who will help me (I have to admit that I'm not among the super pros of Mac, so simple, even simplistic explanations would be great). I'll be very grateful to you in advance, but don't let this take away from your sunny days: I can wait until the rain comes back! So see you soon (I'm never optimistic about the weather).

I missed you so much before I discovered you.