Redmi Note 4x: Cleared or deleted system default launcher

How did you delete the system launcher in the first place? Is your phone rooted?

If you go to the device’s Settings > Home screen & Recents > Default launcher, do you still see the System launcher option?

One quick way to restore everything to the default setup & settings is to factory reset your phone. Be warned that doing so will delete all your files and data on your phone. Do backup your files and data before doing so.