Stats are stored client-side but treyarch games use a custom anti-cheat called TreyarchAntiCheat that usually bans for modified stats. Even in bo2 that came out forever ago just changing your xp/prestige by a little would get you banned/prevent you from joining games but some experienced people can get around it unless the game is new since manual bans will be high when you get flagged. Divinium/Crytokeys are probably server-sided but with the API you can probably find a way to change it but not something people will likely share publicly since they make $$ off it and you would need to figure it out on your own