they appear to be hydroformed necks a process championed by the Taiwanese industry ( on Taiwan there is great experience with this due, for example, to its application in many metal industry related products ) and certainly not used by the French one.
I don’t know whether the Seles are hydroformed but the Selmer necks were never ( and probably still aren’t) hydroformed.
Hydroforming is very precise but it is an industrial way to produce metal tubing.
The expensive molds that are needed ( you would need one for each model of necks that you seek to make) and high pressure machines are of course something you would need to buy after you have learned how they work in the workshop. That’s how, the idea of doing this in a garage on week-ends (which I associate with the concept of DIY) looks jusrt a little less likeley that setting up a proper high tech mechanical shop.
I would be very surprised if anyone attending this workshop could replicate the results without first buying a lot of very expensive equipment .