Gold drops from enemies, chests, and other lootables in the world of Sanctuary. It cannot be purchased or traded and must be earned through gameplay. Gold can be used for things like upgrading your items and purchasing randomized gear from NPC vendors.
Platinum can be earned through daily gameplay activities or purchased with real money. Players can also obtain Platinum by selling items via the Market. Platinum is used for activities such as exchanging goods on the Market or crafting Charms.
Eternal Orbs are acquired with real money only, and can be used for purchasing Platinum, as well as other items such as the Battle Pass, Specialized Reforged Stones, Crests, and cosmetics.
Feel free to checkout the official Diablo Immortal Blizzard Blogpost here.
More Info to come soon - Stay tuned!