The thing is, aside from a few limitations, you really can build your airport however you want. Model a real airport, invent a new airport based on an idea or design theme, or just play as you go - play the way you want. Obviously some design concepts might work better than others - I think this gives the game great replay value - but there’s no “correct” place to put any one element, which is why the in-game tutorial never makes any concrete suggestions.
I like to devise a plan ahead of time and try to build into it to avoid making revisions as the airport grows - something you learn to do pretty well after playing for 2.5 years.
That said, I absolutely agree there needs to be some type of guide to avoid over-building or under-building. Things like how many check-in desks / security / baggage bays per stand at each size, the optimal way to connect a baggage system, etc. At one point I made an airline guide (Airlines and Models) that is now very out-of-date.
I’d love to help build a “Beginner’s Guide.” Ideally this would be something for the wiki, which previously could never be kept up-to-date because of the rapid development of the game. Maybe now’s a good time to try to flesh it out?