Some Outfits are story unlocks, some are so-called Vestige Outfits found from Crypt Sarcophagi, bought from merchants and awarded for side quests, some are from having played / completed the last game Rise of the Tomb Raider, and some are even from the Croft Edition (Deluxe Version).

The Outfits in this game act as lightweight armor with different benefits. Some will give you more resources from hunting while others may give faster health regeneration and some give no benefits at all.

The full body outfits generally don't give benefits (except the 3 from Croft Edition). The Vestige Outfits are the ones with gameplay benefits and after finding them they must be restored at base camps for basic crafting materials. You can loot boxes throughout the game world and then repair the outfits in the inventory. Simply do all Crypts, Side Missions to get them and buy the rest from merchants.