Disassembling Code to Smali
If you want to inspect the app's smali code (instead of Java), you can open your APK in Android Studio by clicking Profile or debug APK from the "Welcome screen" (even if you don't intend to debug it you can take a look at the smali code).
Alternatively you can use apktool to extract and disassemble resources directly from the APK archive and disassemble Java bytecode to smali. apktool allows you to reassemble the package, which is useful for patching the app or applying changes to e.g. the Android Manifest.