Is it possible to configure a smartphone with Android 8.1.0 to power on and off at specified times?
Some OEM ROMs offer such a functionality, most do not. You can check your device's settings, if there is such an entry. If there is none, then it is not possible. So, the answer to this question will be a yes and no together. It will depend on which smartphone you have.
You can certainly not achieve this with 3rd party apps.