What does %Mod{} really do?

Questions & Help Questions
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2 
iex(14)> Map.to_list(%YY{})                
[__struct__: YY, field: nil]
iex(15)> Map.to_list(YY.__struct__())
[__struct__: XX, field: nil]

Kernel.SpecialForms.%/2

Based on the evidence I suspect that the compiled code (it’s a special form after all) sanitizes the value returned from the function by forcing the __struct__ value to reflect the primary module - but that is just a guess.