Depende, pag mod kasi ng app or games need mo i reverse engineer yung existing application manually or by using a software para i open yung existing file na yun. Bale vnviolate mo na din yung terms and agreement per usage ng particular application. Then most cases obfuscated yung code na makikita mo so, mas okay din na manually mo iccode or ggwin from scratch yung program or game para mas customized.