What is required by and for flights that cross borders in terms of border control?
A famous US/Canada example is Niagara Falls.

As as student, part of my cross-country test involved flying a circuit around the Falls, which meant passing the international border.  No special permission was required.

If I'd gone anywhere near the American airport (top right of map), the situation would have been completely different though.

Note that even though there are no specific border control related rules, there are many safety regulations controlling this circuit.  (It's a lot more crowded now than when I flew it.)



GA Flying over Niagara Falls – Fly 'n Things