An interface where you can see
- "everything", or, in other words, all selectable elements, and
- a detail view with more information about the selection
is called master-detail view.
In the specific case of AoE, the large map that constitutes the main game screen is the "master" view.
As master views can be quite extensive, they often provide additional amenities to find specific selectable elements, such as filtering capabilities. This is also the case in AoE, as you mentioned, by making that master view zoomable.
The selection-specific part of the sidebar thus constitutes the detail view. (The sidebar also contains some general content that would arguably count towards forming another part of the mater view.)