While nearly every airline is strictly enforcing on-board mask rules and recent studies appear to indicate that air quality and ventilation on airliners is not a risk for spreading Covid-19, there is no general mask rule for airports.
While many cities and states have mask rules that apply to their airports, many do not, and with that much variation, and that much open space in airports, it's becoming an increasing concern as air travel picks up for the holidays and possibly beyond.
Although the trade group Airports Council International-North America asked the federal government last summer to issue a national airport mask requirement, no action was taken. While it is likely that the incoming Biden administration might respond differently, in the meantime airports are largely relying on signs, announcements, and hopes that travelers will have the sense to be careful.
In the meantime, airports are also testing technology that identifies what surfaces passengers have touched or used and can target aggressive cleaning to those areas. Using 'lidar,' or live visual scanning, passenger movement is tracked and the cleaners can follow. The data is also used to identify areas where too many people are gathered and staff can head there to request social distancing.