What's the difference between going abroad and going abroad?

If an American is going to Beijing from New York Airport via Hong Kong, he has gone through the exit formalities at New York Airport, but this is not going abroad because he is still in the United States. If the registered country of the flight is the United States, then he has gone abroad from the moment he arrives at the Hong Kong airport and walks out of the cabin. Because the flight is an extension of a country's territory, then his next connecting trip cannot be called connecting to a third country or going abroad, but should be called transit, because Hong Kong, Macao and Taiwan and the mainland can only be called transit because of one country, two systems.