I’m useless at modelling so I don’t do that. I really need to learn it. Unless someone gives me a model everything I use is available as a ready item in the SDK.
The time is in placing things and getting them right. This for instance
All of those concrete posts are flowerpots turned upside down. You also have to detach them from the ground so you can control the height and also flip them. The airport isn’t level so each one has to be tweaked.
On the multi storey car park the surfaces aren’t solid so every car has to be individually raised and the height gotten right.
Then there’s painting ground markings, getting the aprons with the right amount of opacity, falloff and fit etc etc.
If you have to do some terraforming then that can often make another area unusable so you have to play around and balance things until it settles down.
If it was just a runway and a few aprons that’s a rapid throwdown. It’s the noodly stuff that eats time. Not sure anyone else cares. I do.