I see a possible source of confusion here.
Say you have a house (a structure) and you build an attached carport. It will require some fire rating of the adjoining house wall, but I don't think the fire rating necessarily reaches the standard of "fire walls with all openings therein protected by approved fire doors". In any event, for the purposes of discussion, say it doesn't.
Now, do you have one structure or two? I say you have one structure, you just made it larger. It was one building before the carport, and it is one building after the carport. From this point of view, every structure "stands alone", in that it eventually stops and is surrounded by non-structure. So every structure contains at least one building.
Alternately you could say that the house and carport are two structures. That would mean that the carport is a structure that doesn't stand alone and doesn't meet the NEC definition of building. The problem with this idea is that it also implies that the house is no longer a building, which is nonsensical.
The conclusion is that under the NEC definitions, you can't have a structure without having at least one building.
Cheers, Wayne