> is Low and High used as an indication of lower input voltage and higher input voltage ?

Yes, but some logic families also have current requirements. With TTL, for instance, a logic 0 is not only a low voltage but the driving stage must be able to sink 1.6 mA of current from the input.

> And if Zero used as a low input voltage then isn't no input can also be considered as Zero by default?

No, because "no input" doesn't always define a voltage. An input that isn't connected to anything is called a "floating input". In the case of CMOS logic the voltage on such an input can be easily influenced by nearby electric fields. Therefore inputs to digital logic gates should always have a well-defined voltage.