This is the most likely way to create workouts. In short, as you create workouts for your athletes, you may need to Save them to your Library, in order to reuse them later. 

Steps, in brief:
1. Drag-n-drop a workout on your athlete’s calendar
2. Modify as needed 
3. Right before you save it on the athlete's calendar, enable the checkbox to save them in your library, as well. 

You can find out more about it here.

You may also be interested in the following: 

1. Creating and saving a workout directly in your library
2. Creating and saving a workout in the library, while working on a training plan (you are reading this)
3. Saving an existing workout from an athlete’s calendar to your library
4. Modifying and saving an existing workout from an athlete’s calendar to your library
5. Using a saved workout from the library on an athlete’s calendar
6. Editing a workout in the library
7. Deleting a custom workout from the library