I am trying to reproduce and modify the mask effect found in this example from the ArcGIS JS API:
There is a lot of fancy footwork to modify the underlying tile layer and apply the mask effect based on a polygon feature layer. One thing I cannot seem to understand is how the underlying tile set is being called, and how I can change it. It seems that this example is using the esri terrain / world imagery tile set, but I don't see where that is being referenced. I want to try to recreate this mask effect using a different layer underneath, but I can't seem to find where I might reference a different layer to use underneath the mask. What am I missing?