Funnily enough i used to weld them together, here is how it was done:
1) Sheet steel would be placed into giant hydraulic rollers and rolled up to form a "can" that was tack welded along the lengh and then a root weld put in it once it was checked, that weld was with a Mig welder.
2) Once we had 2 "cans" you would already have 1 can on a set of wheels that turn both ways and was controlled with a pendant. We would have the 2nd can on a "c" hook on an overhead crane, it would be lowered into place. The fitters would then use heavy duty hydraulic jacks and also weld grips onto the can in order to have something for the jacks to push against. Pretty much the can were pushed by the jacks into place and also you could turn the wheels the can were resting on to manipulate these into posistion.
3) Once the cans were together correctly (4mm fault tollerance) a root weld was put on the inside.
4) The wheels the cans were resting were below a giant welding machine called a "sub-merged" arc welder. These would have 8mm wire and weld at 650-700amps. So the cans would sit on the wheels and turn and then once ready you start the sub arc, obvious the circle welds would be done first and on the outside.
5) After a couple of sections were completed the weld would be tested by ultra sound equipment, issues would have to be air arced out or grinded out and redone by usually hand held mig.
Some of the thicker cans had to be preheated before welding as the heat would dissipate too quickly. I think that there is more modern equipment available now to do this but this was back in 2005 when i welded my last turbine.
I've built them too. The guy who lays out and cuts and bevels the shell plate and the guy who operates the plate rolls share the two hardest jobs because these arent regular cans. They're cones, albeit with a very gentle taper sometimes strapping only a 1/4" smaller on one end than the other. A 4mm tolerance for high/low seems pretty sloppy to me.