Assuming we are talking about an era when Sol has a thriving space industry and the Solar system is broadly colonized. Current materials science supports structures up to 8 kilometers in diameter, and if large scale graphene production is possible, up to 100km in diameter, at least according to Isaac Arthor.

I am wondering what resources would be difficult for a colony ship to reproduce in-situ on an one way trip to the first interstellar expansions of humanity. I picture a true generation ship might be primarily designed around the transport of some of the largest prefabricated sections of a future centrifugal spin gravity habitat.

1. Using hard science to speculate, what types of materials and components would only be available with the massive industry present in humanity's original home?

I picture the main outer ring frame structure of an O'Neil cylinder, like some kind of curved beam, would be prefabricated and sent in a few pieces for later assembly. If the O'Neil cylinder was to be 8km in diameter, 3 pieces would make the generation ship at least 5.7km long.

2. What is practical to transport assuming fusion is in the cards, as are self replicating drones for resource extraction in a region like the astroid belt, and assuming planets are resource poor gravity prisons we avoid in favor of mobility?

3. How might carbon get utilized for large structure fabrication in space as far as processes?

4. What about metals and space based fabrication. How can you picture the production happening in ways that would only be possible in a highly advanced space based economy?

I know this is highly speculative and I hope the mods will let it fly to ask this. I know most nerds are curious about this kind of thing. I'm only interested in the most conservatively realistic of hard science fiction/futurism.