I used to work in the extrusion industry so Iâ€™ll take a quick stab at this. All the following is a best guess and absolutely no facts are implied. In the extrusion industry thereâ€™s a process called â€śover-the-coreâ€ť extrusion, where material (plastic) is fed through a die where the material is formed/shaped around the core (in this case it would be the copper wire). In the case of Romex, itâ€™s probably a two-part process. First, the insulating material (nylon?) is formed over the copper using the â€śover-the-coreâ€ť extrusion method. Exiting the extruder the wire goes through a waterbath to cool and cure the material, and is then fed onto a large spool.
Once the individual insulated conductors are on large spools, they are fed into a secondary extruder. The material that makes up the outer jacket has a lower melting temp than the insulating material, so the insulating material maintains its integrity and doesnâ€™t re-melt.
As far as keeping everything â€śneat and cleanâ€ť within the jacket, from an extrusion standpoint itâ€™s relatively simple with guides, fixturing, and a properly designed profile die. To give you some idea of capability, we used to manufacture medical tubing with an OD of .0785 inches that was fed over a copper core which was later removed. We were able to control the thickness of each individual wall of the tube to plus or minus .0004 inches.
Hope this isnâ€™t too confusing, and again, no facts are impliedâ€¦â€¦â€¦