In reality, it's portrayed by the belnord in manhattan. It serves as the storytelling centerpiece for only murders in the building ’s fifth season —and offers a glimpse into new york's architectural past. The architecture of the arconia/belnord is a perpetual marvel throughout only murders in the building, and injects elements of grandeur and drama to the story.
‘Only Murders in the Building’ NYC Pop-Up Set for August 19-20 – IndieWire
While the arconia apartment building is a key character, there are plenty of other 'only murders in the building' filming locations in nyc you also can visit irl.
On new york’s upper west side, beneath the fictional arconia building, is a secret space
The velvet room—an opulent underground gaming parlor hidden below the building's courtyard