You don't build a rigid shell of course, you build a swarm of free-floating satellites in a range of orbits.
See https://www.aleph.se/Nada/dysonFAQ.html#ENOUGH for numbers.
Do tell.
The manufacturing scale comes from designing factory factories. They aren't that far in the future. Most factory machinery is made in factories which could be entirely automated, so you just need some robots to install machines into factories.