For a very basic outline of what happens on HF, scroll down this page to the "Radio Propagation at HF frequencies" section.
ACMA - Radio propagation fact sheet
As I said - basic, but this may be helpful. HF propagation can also be affected by the conditions of the ionosphere which takes us into yet another set of considerations (We won't go there this time).
In the real world, the earlier comments by Tarq57 can be followed with confidence, especially with reference to the published lists of ATC frequencies used by control centres.
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