No difference between iPhone 7 & X except latter has wireless (inductive) charging as an option.
What do you mean by "voltage" here? Do you mean the AC voltage that the charger receives from the wall outlet? I ask because all USB chargers produce the same output voltage, by definition.
USB standards dictate that the voltage presented to devices for power be 5v. How much current each device draws is up to that device, until USB limits are reached. A USB charger that produces more than 5v is in violation of the standard and may damage connected devices.