Using Flightplan you can take off and land anywhere, so basically the world is your airport. You can set your aircraft to spawn floating on a lake or bay, or sitting on top of a sky scraper. But you can also add a legitimate airport on-the-fly in Flightplan. Just look up the ICAO code (optionally) and drop a pin on the map where you want to spawn to create your airport.
Check out the video above to learn how, as well as the other videos in our How To playlist on YouTube at https://youtube.com/@Simflightplan