3D modeling, or creating 3D custom objects in solid modeling programs like Blender, SolidWorks, Catia, Pro-E, etc., is possible. When it comes to X-Plane, Blender is typically used (some use Sketch Up, but Blender seems to be the preferred program to use for this application). This allows one to create complex, realistic objects like custom terminal buildings specific to a certain airport, ground vehicles, jetways, basically anything. This can be very time consuming task and often requires a lot of dedication when making realistic renditions of a large airport; nevertheless, some of the highest quality payware and freeware airports use these custom 3D objects made from scratch.
Please note that there is no huge shortcut at this level of development. It takes time, so don't be frustrated if this process takes a while for you to master!
Unfortunately, I have not found a comprehensive tutorial that will hold your hand through the entire process of making an airport like, say, Jan Vogel's WED tutorial videos.
Your best bet here is to learn how to use Blender to create basic objects, then use your knowledge to apply what you know to terminal buildings and other objects. There are several tutorial videos, but here is one playlist you can use to get started.
https://www.youtube.com/playlist?list=PLs2aOcA-EaLNX5j2yxVQhEBpFgD3zDR9P
Here is a tutorial where someone has created a hangar for FSX. Even though it is not for XP11, many of the concepts should still apply since the point of the video is making an object in Blender, so it is more focused there than on any one flight simulator.
https://www.youtube.com/watch?v=fVd-RYRnv60
https://www.youtube.com/watch?v=V0pobHrePG8
If you would like me to add anything to this thread, please let me know. This is a work in progress, and the purpose of this topic is to give you a high-level overview of Blender, and to get you started with learning how to use it.