If I use ArcGIS Desktop applications like ArcMap and ArcCatalog, at the ArcEditor/Standard, ArcView/Basic or ArcInfo/Advanced license level, is it using ArcGIS Engine under the hood?
I think the answer is no, but please enlighten me.
ArcGIS Engine Applications that you build, and ArcGIS Desktop applications that Esri builds, both use ArcObjects under the hood.
However, Esri did not use the controls in ArcGIS Engine to build the Desktop apps. For example the TOC Control in ArcMap, while it looks like the one found in ArcGIS Engine, is really a different animal.