DJI video explaining how the "no-fly" zones are implemented:
https://www.youtube.com/watch?v=YoXAMRQoIAA
Category A (large) airports have a 5 mile/8 km zone defined around them in the drone's firmware (but only if you have downloaded a firmware update). The drone is height-restricted between 1.5 and 5 miles from the centre of the zone and will not fly at all closer than 1.5 miles.
For Category B (smaller) airfields it's a 0.6 miles radius no-fly zone.
A downloadable list on the DJI website shows 175 Category A airports worldwide and 513 Category B.