The fog is placed over the world map using a simple Image widget in the container canvas of the world map.
Location:
Map Navigation > UI > WorldMap > WBP_WorldMap
Look for the widget "FogImage"
Note the FogImage has a ZOrder=1, icon that are added as ZOrder 0 will be below the fog and icon that
are ZOrder 2 will always be over the fog(that is always visible even undiscovered area)
An icon covered with fog cannot be interactable with the mouse(hover tooltip, SetWaypoint)
unless that area is explored or set as OverlayFog.