If it's the US, you enter the country when you make physical contact with the "customs territory of the United States." For 99.9999% of the US, that's when you physically enter US airspace.

Where CBP sets up a physical entry checkpoint is purely a matter of architecture, particularly with Global Entry and the fact that passports are rarely physically stamped anymore.

How other nations handle it is a matter of each nation's law. Occasionally there are disputes about which nation owns which piece of property, but that's the general case.

As a USN, if you don't present a passport, you will eventually be admitted because the US can't deny entry to its own nationals, but the more practical point is that you won't likely have been permitted to board by the air carrier.