As defined in \steamapps\common\Sid Meier's Civilization VI\Base\Assets\Gameplay\Data\Civilizations.xml:
Russia has a mild tundra bias (compared eg. with Nubia towards desert).
Arabia has no terrain bias at all.
You can change <StartBiasTerrains> to your likings.
Bias means higher probability. The nature of randomness dictates, that even VERY improbable results must happen ... (
for example loosing two 2:1 battles in a row against an AIplayer ...)