The documented way to loop a YouTube video is to transfer it from this URL format:
Bike Corridor Story Map - YouTube (sorry, it won't let me paste without auto-changing it to a hyperlink. You'll have to right-click to capture the URL.)
to this format:
Both work in a browser, and the second one will autoloop. However only the first format (/watch?) is accepted in the Cascade template (not /v/). I know that there's documentation about how to do autoloop with Vimeo, but I work in local government and we use YouTube, not Vimeo for our video hosting.