This is an easy-to-understand article on how the different forms of radio are used for different purposes. It's based on the same experimentation ham radio operators have been doing for 100+ years.
The CPDLC data link is merely a digital mode much like JS8 that ham operators use daily around the world (weak signal text messaging) and the same goes for communicating via various satellites to other distant stations. Pilots are not going to be bouncing their signals off the moon though as the moon is not reliability in place when required 
See How pilots communicate with ATC while 35,000 feet in the air
When you’re in a sealed, pressurized tube five miles above the ground, being able to communicate effectively is essential. In the early days of aviation, flags and light signals were used before designers were able to fit basic radio equipment into aircraft. Modern aircraft now have an array of communication devices from the rudimentary HF …
source https://gadgeteer.co.za/can-you-hear-me-now-how-pilots-communicate-atc-while-35000-feet-air-its-all-radio
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