Fight bosses, inspect the reward, then verify the recipe in game
The confirmed loop is simple: bosses provide relics, and relics are used for crafting. Anything more specific needs a visible current reward or recipe screen.
The Bosses Update description says bosses use powerful custom attacks and reward relics for crafting new powerful objects. This establishes the purpose of the system, but it does not establish a drop table. Treat any exact percentage, recipe, or “best relic” claim as unconfirmed unless it matches the current game UI.