There are a lot of civilian aeronautical communications on what is known as the "Airband". These include both voice and data. All of these, including the digital ones, i.e., ACARS data are easily monitored on VHF band, spanning:
    → 118.0MHz to 136.975MHz.

Military air communications take place in the upper VHF and into the lower UHF bands:
    → 243MHz to 383MHz.

There are a lot of services and frequencies that have been allocated because these have been developed over many decades.

Most civilian voice communications occur in the Airband and are easily monitored as they are unencrypted AM voice transmissions. The digital communications services are some of the most interesting, like ADS-B and UAT which operate at 1090MHz and 978MHz. These developed later are easily monitored with readily available SDRs like an RTL-SDR or a (similarly priced) specialized SDR like the FlightAware Pro Stick ADS-B SDRs. You can use one of the many open-source applications available for the purpose.

There are also decoders for ADS-B in the OpenWebRX+ application on the STEM Experimentation Bench.

It should be remembered that communications from high-flying aircraft will be received at much longer ranges than their ground-to-air counterparts.

References:

• Aircraft Communications Frequencies  🔗 RadioReference Wiki Aircraft
• FlightAware Flight Tracking  🔗 FlightAware