Departure tax is included in the price of a ticket.
The USA has more than 375 domestic airports, but only a baker's dozen form the main international gateways. Many other airports are called 'international' but may have only a few flights from other countries – typically Mexico or Canada. Even travel to an international gateway sometimes requires a connection in another gateway city (eg London–Los Angeles flights may involve transferring in Houston).
The USA does not have a national air carrier. The largest USA-based airlines are American, Delta, United and Southwest.
International gateway airports in the USA: