Danke! Hat mir sehr geholfen...

They can just walk over the international zone. But if you want it to look realistic, then you can also place a door.

is that "staff can cross" just an open gap, or door?

Damn thats interesting

I know that your passport gets checked at ticketing/security, but non-international passengers without a passport can still enter the area.

They don't have it the same way as for the arrival. Passport or ID gets checked with the ticket before you are able to enter the security screening.

There are so many different concepts all around the planet. Almost impossible to cover every logic in a game. :)
In this game, it's more the European standard where you first are in a domestic/Shengen area and then you enter a separate area for international gates.

While they do have international terminals, I have never seen one that requires a passport to enter/exit.

Actually you can do that, just costs a bit more passport checkpoints and security staff. Passengers only head to the zones they need to. But passengers will directly go there and ignore retail outside the int. zone.

But all the big US airports I know do have a full International terminal, so passport check is very early even before baggage control.

Neat, but usually (at least in the U.S.) each gate has its own immigration/customs (where the passport checkpoint is) rather than an entire terminal being an international zone.

toilets are missing :D