Widgets are the separate parts that make a watch face.
Adding a new widget will give you the following choices, though some of them are simply just other Widgets with a certain prefilled tag or expression:
Just click the plus button in the editor.
Widgets are placed in layers on top of each other. No widget can have the same level as any other widget.
The level of a layer is determined by the order in the widget row below the watch face in the editor. On the left is the lowest widget, on the right the highest. The screenshot shows three shapes you can easily see how they overlap each other.
To move a widget up or down you need to move it in the widget row:
Removing a placed widget is easy: