How could humanity build O'Neill cylinders?

1why space habitats

Humanity wants room to grow, but planets are small and hard on rockets. A giant spinning can in space could hold a whole valley of people.

2Gerard O'Neill

Gerard O'Neill, a physics professor, worked out in the 1970s that free space — not planet surfaces — is where big cheap structures could really work.

3gravity wells

Rockets are expensive per kilogram, and planets have deep gravity wells. But asteroids and the Moon have weak gravity — cheap to mine, cheap to launch from.

4the real dream

So the real dream isn't rockets full of colonists — it's a supply chain of raw material from the Moon and asteroids, built into cylinders using solar power and orbital factories.

5foundation set

That's the foundation: a shrinking Earth, a bigger canvas in space, and cheap material from low-gravity neighbors instead of hauling everything up from home.