@TimBateup ...
My apologies...I was responding with the thought that those collection folders in Studio were within that "My Library" drop-down. I just tested this. When I click on the "Studio" icon on the left-hand global navigation menu, I have a collection folder of YouTube videos for board game tutorials from a channel called "Watch It Played".

Then, when I create a brand new page in my sandbox course, I click on the "Studio" icon (for some reason, my sandbox interface still has the old "Arc" name ... which is what it used to be called before it got renamed to "Studio"). But, as you can see, if I remain on "My Library" and just scroll down a little bit, my "Watch It Played" collection folder is right there. I can click on it to access all the videos within that collection folder.

Out of curiosity, how many videos do you currently have in your collection folder? Even with just one video in the collection folder, it would still look something like this when trying to select it within your content page:

I hope this will help!