Yup as Csgill75 says... if you've ever opened an .airport file in a text editor, you'll see that they are nothing but object placement hierarchies.
These files are also rather small and easy to transfer during multiplayer.
Your machine gets a copy of the custom field in memory when playing on-line, but the file is never written out to the hard drive.
There is NO 3D content in those files, just a list of things and their location in the 3D or 2D world.
The actual 3D objects originate from your own system.