If you want to modify the code in the app logically, you need to decompile it first, and then you will get the unsigned project source code. After modifying the logic, you can use the signature tool (many online users recommend using 360 encryption, which has the signature tool). You can use it normally after signing it. There are still some problems, because there are too many pirated software and the technology is constantly updated. Now Android developers will encrypt or confuse it, resulting in decompilation failure. In this case, they usually give up.