After I extrude the plane (with 0) and take focus off the mesh, I cannot re-select the edge loop again. I believe it won’t since both planes are coincident.
I can not make an edge loop either, maby I made a mess of the object. Still I could shift-select the inner part en extrude it downwards. Took me also ages to grasp there is a full sized face, and that I dont want to remove the inner 0-extruded edges.