Building an airport is complex, even if planes don't need anything to travel between airports, airports take a lot of place because of their complexity.
To move on ground a plane needs a special way : the taxiway, it's built like normal ways but it can't be build on slopes, that's why an airport needs to be on the same layer. To take off or to land, a plane needs a runway : a way which can be only straight. The taxiway is usual way, you can place stations (aircraft loading area), depots and signals to organize the airport. This is a basic airport :
As said above, it's possible to build complex airports. You can build several loading areas, if one is busy planes will go to others by themselves. To organize an airport you can use signals to make one way taxiways as here : a runway is only for taking off and the other only for landing.
To build a realistic airport, you just have to add halls, terminals, control towers, etc …
