Each code varies, some times directly to the name. One of the devs posted FRUITLOOPS in chat once and it was three of all six minor trait stones, 2000 gold, 100 souls, and a gem key. Incidentally that 2000g, 100s, gem key seems to be the default that extra random stones, gold, maps, ect get added to, so that is about what you can expect out of a code.
They used to be in-game, you would redeem them in the store tab, but, well, I’ll let the devs tell that story. Sad News... No More In-Game Code Giveaways turns out giving things away on your website isn’t ‘in-game’ so here we are.
When I’ve gotten them they’ve either been from devs in chat or Tacet the Terror’s youtube videos, but there are more sources.