The area they find themselves in is definitely a military setting. It looks like UNSC Command mashed together with the interior of the Mother of Invention, all metal bulkheads and command rooms and glowing viewscreens. But the place is only partly sketched out. Some of the hallways lead to nowhere, or taper off into foreboding darkness, as if they're a physical representation of the hidden secrets of both places, not meant to see the light of day.

Due to Cortana and Theta's influence, reality keeps unraveling and turning into streams of flowing data and then back to matter again.

Also, there's a random skateboard ramp. That's entirely Theta's doing.