It had to get in there somehow.

Have you removed the doors then taken measurements. All refrigerator door usually come off with a screwdriver or small wrench. Once off you can again measure width and it will easily pass through a standard apartment door and should get down stairs. Again... it got in there in the first place.

Most refrigerators come with the doors off when new and are assembled by the delivery crew as part of set up.

If this is being discarded, local law REQUIRES the doors removed as a safety issue to prevent small people from accidentally being trapped inside and suffocating. If the doors are not removed the doors need to be substantially secured shut and sealed, however that can still sometimes get a safety violation. Always take the doors off.