One way is to start with a flat sheet of steel. This sheet can be pierced through with a mandrel in specific locations and diameters. With no cutting away aspect the steel will stretch and the sockets formed. The sheet with protruding sockets is bent around another mandrel, forming the shell ready to weld. Further steps can dimension the shell, thread/face it and scallop the sockets to a pleasing shape. On some lugs (Bocama and Prugnat as example) you can see the two different lug shaping cuts, creating the shore line. (Blending these two different lug edge contours is an early thing young builders to be did back in the day). So much of basic frame building is just blacksmithy stuff. Metal can be rather plastic if dealt with properly. Think of soda cans that are formed from a sheet. Andy