I found that .dex is compiled android application file and it will used by DVM(Dalvik Virtual Machine) and DDMS(Dalvik Debug Monitor Service).
It means, .dex file not required for physical android mobile?
DEX is the file format for the compiled bytecode that goes into an APK file. It will be used on devices and emulators.
Note that DDMS has not been used in years. I recommend that you check the dates on the materials that you are reading and focus on newer items.