An input pin — our voltmeter.

An input senses voltage; an analog input measures its level, like a voltmeter.
A voltmeter measures the voltage between two points.
The red probe touches the point we want to measure.
The black probe touches GND, our reference point.
Arduino’s inputs share the same GND reference — like having the black probe already connected.

Try it: what does 5V look like?

Connecting a 5v directly to an input - has no meaning.

the "red" and "black" are "connected" through the same "point" (pin).

- try analog read with serial terminal, and cross 5v to input. see what happens.