A piece of steel round stock is cut to length and bored for a large, long Allen head bolt. A channel is cut into it for for a Woodruff type key to hold a pulley for the toothed belt. Pulley also uses set screws (to lock in) once the pulley is lined up with the receiving pulley The original crank snout is cut down and..I imagine ....some of the snout is left for the bolt to screw into and to stabilize/center the extension. The extension end which butts against the crank would have to be machined for this.The bolt is tightened, the extension trued and the bolt head is welded to the end of the extension.
I do not know if this is the exact process but it is the best I can think of after looking at the cranks.