Larger buildings with many units usually do have intercoms with two-way audio (and sometimes one-way video) and the ability for the resident to remotely unlock the door. Sometimes these work via telephone or Internet, so the resident does not even have to be home.

Smaller buildings with just a few units usually just have simple dumb wired doorbells. And they are usually not broken.

Different cities/neighborhoods have a different mix of larger and smaller buildings.

This QR code idea is interesting, because it lets you go from dumb to smart, bypassing all of the steps that would normally be necessary (cost, approval, installation, maintenance, etc).

It's not as "good" as Nest etc, but it gets you most of the way there for almost none of the hassle.

Not for me, probably, but interesting.