i know that there can only be one immortal gu each. im at chapter 600+, when fang yuan deduce the ninety-seven or ninety nine recipe to 100% from lang ya spirit he gives it back to to lang ya spirit. he knows now the recipe, can he create it? even though he give back it to langya spirit? other i dont understand is the gu remnant recipe, how it works?
If he has the ability and resources, the success rate is basically close to 0 even if you're Langya spirit. There's two variants. If it's a remnant recipe then it basically means that the materials used for the recipe doesn't exist anymore or it's unknown how to make them, or a portion of the recipe is missing so you have to deduce the missing steps etc.