What's this metal thing in my wall where the studs should be, and how do I put a shelf on it?
Given that this wall separates two apartments in recent construction, the wall you're drilling is likely not to be "normal" familiar construction. It's probably some kind of "area separation wall" designed firstly to limit the spread of fire and secondly to limit the spread of noise between apartments. You might do well to seek information from building management as to the construction of that wall to ensure that you don't compromise its fire resistance characteristics.