My wife and I are flying to Iceland and we are trying to book tickets but I wasn't sure about the flights.
So I am just asking to clarify. If I leave the US through an airport, do I have to return to the US through the same airport?
Your question is well founded. Some type of visas to some countries allow you to enter and exit via specific entry points only. For example, ENTRI visa of Malaysia limits to persons who are appearing from direct flights and go out of country on direct flights (or transit to specific countries). And the Airports of arrival is also limited.
For USA, there is no such restrictions on any passport or visa. You can enter any international airport and use any other mode of transportation and use any border to come in and out of the country. Also, note that US doesn't have any exit checks. So, if you are not a US citizen, make sure that your exit is recorded in I-94. Especially if you are not departing by air.
There is no requirement to leave and enter through the same port of entry. Note that this applies to everybody: US citizens, permanent residents, visitors, and anybody else entering or leaving the US.
If you are planning flights, you are certainly free to choose flight routing that comes back via a different route if that ends up being more economical for you.
For the reason, look at the question reversed. Do you need to exit at any particular location relative to your entry point? No, because the US does not have exit controls.
In fact, if you are not a citizen, you should take your own initiative to document that you actually did leave the country, either by airflight, or with an I-94 form.. Otherwise you could be accused of overstaying.
The first time I was aware of leaving (there was a previous time as a baby I know almost nothing of) the US I exited via San Francisco (with a fuel stop in Seattle) and returned via New York.
Since then I have called a trip short and the outbound was via Seattle and the return via IIRC Los Angeles.