The map maker decides that, cav. There are things called "entries." There's a Fighter, Bomber, Vehicle, and Boat type. By placing a standard "Port" object that has everything preassembled, a boat entry is placed at those little docks. Since other fields aren't expected to be on the shore, they don't automatically have a boat entry. By placing a boat entry right off shore manually, a spawn is created. By placing a boat or vehicle entry out in the middle of nowhere, you create a spawn there. You can even have the air starts over enemy fields if you place a Fighter Entry from your base over the enemy field at the desired alt. That's how the airstarts in the TA and on the Japan1 map work. There are also Fighter/Bomber entries in the spawn hanger and on each of the runways. There's a Vehicle entry inside the vehicle hanger on vehicle bases, ports, medium and large fields. It's slightly in front of the VH on small fields.
