In airport design, do international arrivals mix with domestic arrivals?

International arrivals will have to go through immigration and customs, domestic arrivals do not. That makes mixing them difficult and not very practical especially if the airport needs to support international transit as well.

VIP handling is a different issue. Most airlines will provide some accommodation anywhere from extra (and hopefully shorter) lines for check-in, security, immigration & customs to driving you around the tarmac with a black limo and a personal host that walks you through all parts of the process with high priority privileges.

In most countries, international arrivals requires a separate stream of passengers, segregated from everyone else in the airport. Passengers arriving from other countries must go through immigration and customs checks before they are allowed to come into contact with others, as "undesirable" passengers might otherwise sneak into the country or pass off prohibited items.

In the United States, US Customs and Border Protection publishes an exhaustive document called the "Airport Technical Design Standard" (I'm not able to find a current copy online) that details the physical requirements for handling inbound international passengers. Everything about the environment must be controlled to CBP's specifications, from the corridors to the access control systems to the security cameras. These rules extend to the gate area, who can access the plane and when, the movement of uncleared passengers and luggage, even the disposal of international garbage (some airports incinerate it on site to comply with CBP requirements). There are special procedures for where international arriving passengers are taken if the terminal must be evacuated, procedures for allowing specially cleared airport and airline staff access to this secure area, etc... Other countries concerned about border security have similar requirements. All of this is to say that countries spend a lot of time and resources thinking about the design of facilities that handle international passengers with security in mind. Any design where passengers mix or take unusual paths would violate the technical standards for the design of these secure areas.

In addition, some airports treat international arrivals (or a subset of international arrivals from some countries) as "unscreened" for security purposes and require that they must go through airport security before allowing them back into the terminal concourse. They may not require the same rescreening for domestic connecting passengers. For example, a passenger arriving at San Francisco from Frankfurt must go through all of US immigration, customs, and TSA security screening before they can board a connecting flight. If this "unscreened" passenger were allowed into contact with passengers from other flights, he could potentially pass off prohibited items before going through screening.

It is, of course, possible that a particular VIP might be escorted by airline staff or immigration officials through a special route or procedure. This would require the approval of the airport's border officials, as it would bypass the normal security measures for arriving international passengers.

This really depends on which country you are in and what its laws are. There are three theoretical cases of which I’m the last two are much more common. Note that for the sake of this answer I’m subsuming entities like the Common Travel Area or the Schengen area under domestic since no passport controls happen there.

1. The country either does not check the passports of incoming international travellers or systematically checks passports of its domestic travellers. It either does not care for customs or requires domestic travellers to make a pro forma customs declaration.

   In this case, either everybody or nobody must go through a passport/immigration control and thus both types of passengers can be mixed. They can also collect their luggage in the same areas and proceed onwards to the same customs declaration. Either domestic travellers are also required to make a declaration or nobody makes a declaration.

   While some countries such as Russia require internal passports at least for internal rail travel (but probably also for internal air travel) this is a very unlikely case.

2. International travellers are subject to immigration/passport controls; domestic travellers are not. They proceed to the same luggage carousels and either both types or neither makes a customs declaration.

   If the laws of the country require this setup, it is absolutely mandatory to separate international and domestic arrivals at least until behind the immigration gates. Whether the travellers then mix or not is a different question.

   As far as I know, some European airports use this setup, with luggage carousels being behind the immigration control and everybody making a customs declaration upon exiting. The declaration is made by choosing either the green or red lane and there are large enough signs saying use green if you are from an EU country.

3. The country requires international travellers to subject themselves to both immigration and customs checks; from the way these checks are performed, it is mandatory to separate international and domestic arrivals until the former have made their customs declaration.

   I would guess that this is the standard for most countries of the world. They first want to know who is coming in (do we need to bounce them or are they okay?) and then they want to know what is coming in (do we consider that item legal or do we want to levy some customs duties?). Subjecting domestic travellers to this zealous inspection would either not be well received by them or would be actively inhibited due to security reasons or to prevent customs circumvention.

When you are designing your airport, it is up the the country you are designing it in which of these applies; but my money is on number 3.

Note: This post is only focussing on the question on whether international and domestic arrivals can be mixed on their way from the jet bridges either to the outside via the luggage carousels or to connecting flights. An entirely different story and not covered is the question whether arrivals and departures can mix, and if so which types.