I know the developers said that Customs and Immigration would be added later to the game. But I thought I’d offer my thoughts on it.
Seems to me the solution is to split the “secured area” room into two - “Domestic Secure Area” and “International Secure Area”. Gates assigned to domestic flights would be attached to the domestic secure area (domestic departure hall or concourse) and international flights to gates attached to the international secure area (international departure hall or concourse).
Arriving international passengers are segregated from departing. This would require a way to force passengers to follow a particular hallway (maybe something similar to the secure area exit gates in the videos) to the Immigration hall. Then once through that to a dedicated baggage claim area to pick up bags, then through Customs and out into the non-secured area of the airport.
If there are transferring passengers to another international flight, since 911, they are also segregated but directed to security screening area then from there directly into the international departure hall (e.g. London Heathrow, Vancouver Canada) without clearing customs and immigration.
Note before 911, passengers would get off the plane directly into the international departure hall. Transferring passengers to another international flight could wait there for their flight. Arriving passengers would make their way to the arrivals hall (customs and immigration)…
If transferring to a domestic flight - the arriving passenger would go through immigration, pick up their bag, go through customs then recheck their bag, usually on a “transferring baggage drop” for that purpose just outside customs. Then the passenger goes through security into the domestic departure area.
Looking at the game so far, and from what I’ve read, I think international could be added to the game. Multiple floors would help as it woudl be hard to design an airport to segregate passengers without them crossing over or under each other.