Impulse response from Frequency response why using $e^{j\omega}$ as an input?

1. Why the impulse response can be used to determine the frequency response of a system?
2. What is impulse response how it can be used in signal processing?

Why the impulse response can be used to determine the frequency response of a system?

The impulse response is the response of a system to a single pulse of infinitely small duration and unit energy (a Dirac pulse). The frequency response shows how much each frequency is attenuated or amplified by the system. The frequency response of a system is the impulse response transformed to the frequency domain.

What is impulse response how it can be used in signal processing?

In signal processing, the impulse response, or impulse response function (IRF), of a dynamic system is its output when presented with a brief input signal, called an impulse. More generally, an impulse response refers to the reaction of any dynamic system in response to some external change.