Aeronautical Communications
There are a lot of civilian aeronautical communications on what is known as the Airband. Communications on the air bands includes both voice and data. Voice comms typically use AM modulation and are not encrypted.
Legal Note: Monitoring unencrypted civilian (and military) air communications in the U.S. is generally legal. However, recording and/or sharing, publishing or streaming them is legally questionable and is not recommended.
Civilian Voice Communications
All of the civilian communications take place in the lower VHF band spanning 118.0MHz to 136.975MHz.
Most civilian voice communications are easily monitored as they are unencrypted AM voice transmissions.
There are a lot of services and frequencies that have been allocated because these have been developed over many decades.
Civilian Digital Communications
The digital communications services are some of the most interesting, like ADS-B and UAT which operate well above the Airband, i.e., 1090MHz and 978MHz.
These were developed later and are still evolving.
There are decoders for ADS-B and UAT broadcasts in the OpenWebRX+ application on the STEM Experimentation.
Military Communications
Military air communications take place in the upper VHF and into the lower UHF bands from 225MHz to 400MHz. This is known as the Military UHF Aviation Band.
In contrast to most civilian aeronautical communications many of the military aviation communications will be encrypted.
Monitoring
Both voice and digital communications are readily monitored with SDRs like an RTL-SDR or a (similarly priced) specialized SDR like the FlightAware Pro Stick ADS-B SDR. You can use one of the many open-source applications available for the purpose.
References:
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Information is provided for education and instructional purposes only.