how hackers can hack server sided games

i wonder how hacker are able to hack server sided games like adding credits that usually cost 100 buck in the playstore but they sell it for same amount of credits for 50$ so tht mean somme hackers can get in the game under your account and add the credits . i wonder how they do that is there any way to learn that and what is it call is it blackhat hack or whatever sorry im newbie and i want to learn how to do it

usually they charge from RUS.games and ingame currency in russia is cheap compared to the rest of the world. for ex. in russia Modern warfare cost 30$ the rest of the world have to pay 60$+