More techie fun. It’s actually fairly easy to do, it was the idea that was brilliant (is there an emoji for an exploding head?)
Plex media server has a “watch later” function, that can be used for watching *selected* YouTube videos. See a video while browsing, and want to watch it later? Simply open the video page and click a browser bookmark. When you next open Plex, it will be listed under your “watch later” items.