What is the difference between Input signal and input voltage?
They are not contradictory. "Input signal" is a general term while "input voltage" is a specific term.

In electronic circuits, electric attributes carry information while in electrical circuits they carry energy. Both voltage and current can be used as input signals. So "input current" is another form of "input signal".

For some reasons, we prefer using voltage than current as an input signal. Current interface is an exception.