From AI:

While a common myth likens the presidential aircraft to an embassy—which is considered the territory of the host country—international law does not actually recognize airplanes as sovereign territory of the state in which they are registered.

Instead, Air Force One is considered U.S. property that benefits from extensive functional immunity. This status functions in several ways:

Exempt from Search/Seizure: International agreements, customary practices, and host-nation consent protect the plane from being searched, seized, or boarded by foreign authorities without permission.

Jurisdiction: The United States retains legal jurisdiction over U.S. citizens and actions taking place aboard the aircraft.Because sovereignty is tied to fixed geography, Air Force One operates as an extension of U.S. jurisdiction while flying through international airspace or when parked on foreign soil, rather than a physical extension of U.S. land.

Underneath the "D" in UNITED STATES