That’s the interior of the bulkhead where a stair meets the roof in an old fire-rated building. Too bad the bulkhead’s not rated, as the steel frame (made of angles and channels) that supports the roof and walls is not in any way protected against fire.
Here’s an annotated version to make it clearer:
The angle posts that support the roof are outlined in purple, the channel spandrel beam is outlined in green, and the red arrows point to the bolts that connect the beam to one of the posts. The wall is terra-cotta block, stuccoed on the outside and plastered on the inside. The roof is a concrete slab supported by very closely spaced steel beams – the sprinkler pipe is hung from one of the beams.
This is, by current standards, non-combustible but unrated, and therefore little better in a fire than wood-stud construction. But it was cheap and easy to build.