According to this 2018 article, the top ten biggest property owners in New York City include (among others) Vornado Realty Trust, SL Green Realty, Tishman Speyer Co., Blackstone Group, Related Companies, Brookfield Property Partners, and RXR Realty (see https://ny.curbed.com/2018/9/14/17860172/new-york-10-biggest-property-owners ).
Sociologist Saskia Sassen has done some brilliant work in this area; see, e.g., https://www.theguardian.com/cities/2015/nov/24/who-owns-our-cities-and-why-this-urban-takeover-should-concern-us-all :
{T}oday, rather than a space for including people from many diverse backgrounds and cultures, our global cities are expelling people and diversity. Their new owners, often part-time inhabitants, are very international but that does not mean they represent many diverse cultures and traditions. Instead, they represent the new global culture of the successful and they are astoundingly homogeneous, no matter how diverse their countries of birth and languages. This is not the urban subject that our large, mixed cities have historically produced. This is, above all, a global corporate subject.
....Since their beginnings, whether 3,000 years old or 100, cities have kept reinventing themselves, which means there are always winners and losers. Urban histories are replete with accounts of those who were once poor and quasi-outsiders, or modest middle classes, that gained ground because cities have long accommodated extraordinary variety.
But todays large-scale corporate buying of urban space in its diverse instantiations introduces a de-urbanising dynamic. It is not adding to mixity and diversity. Instead it implants a whole new formation in our cities in the shape of a tedious multiplication of high-rise luxury buildings.
This next article begins by studying changes in ownership in New York City's East Village over a twelve-year period and goes on to mention,
The New York Times article Homes Dark and Lifeless, Kept by Out-of-Towners describes how some apartments of Manhattan, mostly in wealthier neighborhoods, remain empty throughout most of the year.
Since 2000, the number of Manhattan apartments occupied by absentee owners and renters swelled by more than 70 percent, to nearly 34,000, from 19,000,...
and
The International Consortium of Investigative Journalists also touches on the challenge of tax avoidance in Hidden in Plain Sight: New York Just Another Island Haven as high-end New York real estate is an alluring destination for corrupt politicians, tax dodgers and money launderers around the globe because of state and local policies that lavish tax breaks on Manhattans wealthiest homeowners and federal policies that allow real estate agents to close their eyes to whether their clients are trafficking in illicit money.
The article also mentions the process of layering: creating mazes of bank accounts and offshore companies to move and hide money in which if they are laid down skillfully, its often impossible for authorities to detect flows of illicit cash. Towers of Secrecy also went through how hard it was to investigate and find out who were the actual people behind the properties. LLCs, even if not created for those specific purposes, becomes a tool for some people to hide their secrets beyond just to remain private. They remain out of sight so their activities do so as well.
More at
https://disquemechudo.github.io/who-owns-nyc/data-project-final.html .
Anyway, yay for Spain!