From Halopedia, the Halo wiki
Skydocks are a kind of structure built and employed by the Forerunners. They serve as floating docks, presumably for smallcraft such as Phaetons. They are capable of maneuvering, and utilise torsion plates to achieve this.
Halo Infinite concept art for what appears to be two skydocks, or very similar structures.
- Halo Encyclopedia (2022 edition), page 338