That’s actually pretty hard to answer... In Just Cause 3, Rico came with one of the planes you are talking about, but it took off on an aircraft carrier and we didn’t saw the plane land as Rico felled off. There is a small civilian airport/airbase in Insula Fonte which have too short runways. There is an unmarked airfield down the military port in Insula Dracon which have ond long runway (note that there is also an runway at the military port said above, but you have to drive through the military base to get to it) and finally there is a big airfield south of the trident port in Insula Striate. It look like a civilian and military airfield, with a long runway and a fiew civillian planes, but as you said there is no Terminal, except the one building near the control tower. Also I am sure people get check there paperwork when getting out of the plane. Also i am sure that there is probably people that come in Medici by boat, but I am not very sure how it would work...