I can see that there are a lot of tutorial on creating widget. But, Is there a tutorial in Apple on how to create these widgets?
Widget - Customize - Is there a tutorial in Apple on how to create these widgets?
I think those are older style widgets, not the new ones using WidgetKit.
That "Customise" area only appears in edit mode around the older style widgets, so in this case it looks like the "Workspace" app hasn't been updated to use WidgetKit.