Atlas is an Esri Experience Builder application.
An Experience Builder application is comprised of one or more "widgets" that can communicate with one another and can be assembled to form an application.
The web map that you work with in the Atlas application is a standard Experience Builder widget, and the Atlas project wizard itself is just a custom widget that interacts with the map widget.
Currently the Atlas application is assembled with several standard "out of the box" Experience Builder widgets, along with the custom Atlas Project Wizard and Metadata Layer List widgets.
The Atlas Project Wizard widget is docked on the left side of the screen in a re-sizable panel.
The following widgets are accessible through their icons in the top right corner of the application.

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