A widget is a configurable functional unit. It is the building block used to create pages for an experience. ArcGIS Experience Builder provides OOB widgets, such as map, button, list, etc., and custom widgets can be created. A widget usually provides a setting UI to allow users to configure its functionality in the builder environment. If a widget has configurable options but does not include a setting UI, a JSON editor can be used to configure it.
Learn more about widget implementation.
In the app config, the widgets are defined by widgets.