An exercise is the smallest unit in the training library: a name, a description, and usually a diagram. Sessions are built from exercises, so anything you want to reuse belongs here.
Go to the Training library, click Add content and select Exercise.
Fill in the Name, the Description and the rest of the details. Paste YouTube links straight into the Description — that is where video demonstrations live; there is no separate video field.
Add a drawing or animation with the drawing tool if the exercise needs a diagram.
Add Tags. Tags are what makes the library searchable once it is more than a handful of exercises, so it is worth tagging as you go rather than later.
Optionally file the exercise in a Collection. A collection is private, shared with the team, or shared with the club, and that choice is the collection's, not the exercise's.
Finish. The exercise is now in your library, and in the collection if you chose one.
An image added as an attachment becomes the exercise's thumbnail automatically, so the first image you attach is the one that represents it in the library.