From looking at pre-collapse photos, it looks like the pool may have been above at least part of the parking garage. That is a common design in the area because the sites for these kinds of buildings are usually too tight to allow enough land for a pool on the ground. I’ve been in other buildings in the area that have the pool above the garage, with the requisite heavy structural members in the garage to carry the load of a pool filled with water above. Taking another look at post-collapse photos, the pool is still recognizable and may still be at least partially intact. Perhaps it is cracked and not fully caved in. Like so much of this situation, many unanswered questions.