Where android apps are stored?

Where an app is stored very much depends on several criteria:

• System apps / pre-installed-bloatware-apps are stored in /system/app with privileged apps in /system/priv-app (which are mounted read-only to prevent any changes)
• normal apps in internal memory go to /data/app
• some apps (encrypted on internal storage?) go to /data/app-private
• Apps stored on external memory go to an encrypted container in /mnt/sdcard/.android_secure. As at runtime Android needs them to be decrypted, it will decrypt them and store a decrypted copy on tmpfs (so it’s gone on a reboot) in /mnt/asec
  (you cannot simply look into /mnt/sdcard/.android_secure directly from the device; but if you use a card reader and attach the card to your PC, you will see the files there have the extension .asec instead of .apk — from which you will get the connection to the name /mnt/asec).
• the apps data are stored below /data/data/<package_name> (internal storage) or on external storage, if the developer sticks to the rules, below /mnt/sdcard/Android/data/<package_name>.

Source.