From Halopedia, the Halo wiki
UNSC facilities existed above Reach designed solely for the construction of military vessels in vacuum. Six of these were placed near the refit station Anchor 9. During the Fall of Reach, the six were still constructing heavy frigates for the war effort in various stages -both the ships they were building and the platforms themselves. To construct the ships, each platform had a pair (or more) of cranes attached to railings, allowing them to fit large floating sections into place.