How to find and merge diffrent parts of the building, if there is no building relation tag. For example this one: 120 Wall Street (https://www.openstreetmap.org/way/278076374#map=19/40.70476/-74.00595)
Short answer …
Find: All bullding:parts within the building shape belong to the building.
Merge: Not necessary for 3D modelling and not commendable due to possible side effects.
Building relation tags: Not always needed.