To add widgets to items in the list, go to the customizer preview, and click at the bottom of the editable area corresponding to an item.
Click on ADD ELEMENT and select the last option (Widgets Area).
Next, go to the WordPress Admin Dashboard -> Appearance -> Widgets.
Drag and Drop, from the available widgets, those that you want to include in a certain widget area.
In the customizer preview, select the Widgets Area:
The added widgets: