They mostly don’t? I’m sure there are a few, but the vast majority are financed, built, and operated by corporations.

I don't know enough about the US market to speak too strongly here, but I'd have to imagine based on my experience elsewhere that if you wanted to buy a flat from a property developer right now you could.

I would have thought that "build to let" would be the rarer case, but like I said I might be blind to the idiosyncrasies of the US here