Good morning! To add airports to the game, you will have to copy txt files from your pc to the /locations folder of the game. A good first step is to download airport files from this page (locations.zip, see the download button) and copy one raw airport txt file to the game (for example choose SBGR_gen.txt if you want to add Sao Paulo airport). If the file is in the right location, then it should appear in the game (in the airport menu after you started the game). Then, you could step by step add more details to the txt file, by using the example.txt file as a reference. This can be complicated, but you can also check gitHub or discord to see if someone else has already created a file for the airport you want to simulate.
Hopefully this is enough info to get started with creating airports. Good luck!