With a few keystrokes, he injected a modification. Instead of checking if gemBalance >= itemCost , he flipped the logic. Now, every time he "bought" something, the game added the price to his total instead of subtracting it.
If your goal is to modify the logic of the app, you cannot simply edit the APK like a zip file. You must work with (the intermediate language). Android Studio Apk - Mod
Use a command-line tool like Apktool to decompile the APK into Smali code. With a few keystrokes, he injected a modification
After signing, you have a ready for installation. With a few keystrokes