How can an iPod touch play mp3 transferred as files on storage instead of Apple sync using iTunes?

The native app for music playing on the iPod touch assumes music files are stored in a database that indexes their local storage in a sandboxed manner. Since it was designed to only receive songs by syncing with itunes software directly from Mac or Windows supported OS, it’s not clear there is a general folder that gets indexed or processed for playing like some other apps that expect to be thrown a folder of files to play.

For Linux, you should use an alternative software "rhythmbox" to accomplish this import to the database of playable files.

If you use ifuse, it will only transfer the songs like an usb drive for storage only and not for playing. This mirrors the storage function of the original iPods where you could dump files, but they’re not indexed or processed for apps to use. The files app for iOS came after your version of iOS so this function to store and play arbitrary files came later than iOS 6 and isn’t available to you. Perhaps others know a player app that might be able to play these or jailbreak your device to add this function.