I've seen a bit of milk plant SS pipe welding done. When the professionals I have seen can do it, they tack it and then rotate the pipe in a roller stand while welding it. That seems to be the easiest way to achieve the best weld by hand. The joints that cannot be rotated would be a greater challenge. Also no filler is used. Otherwise sounds just like what was described above as far as torches, tungstens, purge and flow rates. They used a borescope to check both old and new welds. At some point there would always be a weld that couldn't be reached to check it, so I suppose you just have to rely on the welding procedure and skill of the welder.
The most recent large scale fabrication I know about was done with the automated orbital welding machines.
I would pay close attention and ask lots of questions whenever the hired welders come in. I think it is a great idea to learn the skill.