Just checked out FlightRadar24 and the C-17 is not showing up on their radar...lots of civilian/commercial. I do like the interface though.
These aren't ARTCC "Center" transmissions you are hearing from air traffic controllers, but ARINC relaying ATC clearances.
Probably more military aircraft into Elmendorf AFB then commercial planes into Anckorage International Airport.
I was an Air Traffic Controller for over thirty years. I worked at New York ARTCC for eight years until 1980. We had hf radio to transfer non radar oceanic traffic to Sana Maria ATC. I don’t know what they use now. When propagation was bad we used teletype. After 1980 I spent twenty years at BWI Tower. It was closer to where I grew up. AIRINC relayed clearances to aircraft out of VHF range via hf.
None of them really have decent HF antennas though. You're usually hearing 400W (output) transmitter connected (via an automatic coupler) to an antenna embedded into the tail. None of the (ARINC) HF operators are actually "Air Traffic Controllers" They simply relay the position reports, requests & clearances etc to the respective ARTCC (Air Route Traffic Control Centers) and then relay the reply. I'll also add that CPDLC is literally going to make HF SSB for aircraft position reporting, clearances and other aircraft "traffic" "obsolete" in that it will eventually become secondary and little used once CPDLC is fully implemented. The aircrew will still likely do a radio check on the ground prior to departing by actually be transmitting "in the blind" and hearing "sidetone" (in the headset) and then assuming it's "good to go" It's actually the other way around. Elmendorf is not all that active an "airport" compared to Anchorage.
It probably seems counter intuitive but altitude past a couple of wavelengths doesn't seem to help at all unless you're trying to work line-of-sight like on 10m. High altitude (like altitudes in the 20,000ft-40,000ft) on 75m or 40m for example doesn't seem to make any difference at all!
Lately I’ve been making a habit of checking Shannon VOLMET on 5505 kHz. It’s a usually pretty good indicator of propagation to EU on 80m.