how op amp differntial input is zero?

The differential voltage between the inputs can be approximated as zero, if the opamp is in a stable. negative feedback situation, because its gain is very large. only a differential voltage very near zero, multiplied by a very large gain could produce an output voltage between the power supply rails.

The input impedance is high for very independent reasons. Some opamps do not have high impedance on the inverting input (current feedback opamps).

Because it uses a long-tailed pair input configuration. With perfectly matched devices this has inherently zero offset.

Do you know how to calculate the input impedance of an active stage ? In which case it should be obvious.

Graham