Smali and DEX

DEX (Dalvik Executable) is the compiled bytecode format that holds an Android app's code, and smali is its human-readable form. When an app is decompiled, the DEX is turned into smali so an analyst can read and trace what the code does.

Working at the smali level is how reviewers find logic, keys and behaviour that do not appear in the original source, including inside third-party SDKs. It is a core part of Android reverse engineering.