I noticed on the chart that not all airports are alphabetic
SAD and TUS, but near me there is also P29, P04, and P03

some of the smaller community airports have numbers in the designators

P29 is a place that's too tough to die
P33 is where the Rex Allen Museum is
P04 is a College air strip

you also find Public and private heliports have alphanumeric designators

JOHN

ah ICAO codes differ from IATA codes

EDDH and EDDF are in Europe

YSSY and YPAD are in the Pacific

see if you can figure out the IATA codes for these