HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

I think I found the answers to the second question. a) The Heroes3.exe executable file appears when you rename the HD shortcut you created, for example. This way, you can set different settings for both files (e.g., a standard balance for the .exe file and a custom map for the HD.exe file).
b) The Russian Mega Pack provides 2 keyboard layouts.
Did I understand correctly?