In the most conceptual sense, a click or keypress briefly completes an electrical circuit. The part of the computer that handles the keyboard sees this as a zero value changing to a one; the computer then initiates whatever sequence of events it’s been programmed to follow to interpret the keypress, send the keypress value to where it needs to go, etc.
Of course, the reality is far more complex: Most keyboards have their own built-in controllers that can briefly store keypress data, and signal the CPU when there’s data to send; coordinating with PC components such as the interrupt controller (which helps the CPU handle things like keystrokes in an orderly fashion); and the keyboard’s device driver, which is running under the control of the operating system. Plus, in most current setups, the USB or Bluetooth subsystems may also be involved, funneling data and/or power to and from the keyboard.
It’s surprisingly complex, once you start to dig in!
If you want the full rundown, here are some good sources that explain things in detail:
Example: What Happens When You Press A Key
What happens from the moment we press a key on the keyboard, until it appears in your word document
What happens when I press a key on a keyboard on a computer
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