I do not speak Polish so I used Google Translate.

An object with layer=1 can of course touch the ground. It is just used to describe the vertical relation of objects, nothing else. Bridges with layer=1 also often touch the ground and don't hover. Do all the pillars of a bridge have the same foundation? I guess not. Are the pillars of a bridge not part of the bridge?

The different properties can be mapped using building:part.

You are saying that there are terminals and jet bridges. Well, the jet bridges are not mapped at all a this airport so what is mapped now must be part of the terminal then.

There are enough images that show how the building looks from the outside. The number of floors inside has no affect on the outer properties. This airport building is not much different from other airport buildings after all.

Bzw. das stimmt nicht ganz, ich habe auch die neue Bahn nicht selber eingezeichnet, sondern nur erweitert.
osm.org/changeset/86053640

Bei der neuen Linie habe ich mich nur an der alten orientiert. Auf mich geht das also nicht zurück. Ich habe jetzt keine Tiefgründige Recherche betrieben, aber das könnte der Ursprung sein: osm.org/changeset/2578178