How does input from IR receiver to Arduino UNO board work?
You confuse the different technologies here.


PWM is only an output, not an input. So every pin marked with the PWM symbol can OUTPUT a PWM signal. Reading a signal is a totally different task with different hardware involved.
The IR receiver does not output a PWM (meaning Pulse Width Modulation) signal to the Arduino. It ouputs the demodulated IR signal.


So here is what happens: The remote a code, that consists of different periods of "light on" and "light off" (giving pulses of different length, which encode the data from binary format). Since you want to seperate the remotes signal from any other IR signal, that might roam through the air, the remote also modulates the pulses with a special frequency (often this is 38kHz, but might be another frequency around that). This means that every pulse will be chopped into shorter pulses at the specified frequency. This translates to something like: "light on and off with 38kHz for duration x" --> "light off" --> "light on and off with 38kHz for duration y" --> ...

The receiver module senses the IR light comming in and will electronically discard all signals, that don't have the special frequency, and does the demodulation on this. The outputted signal does not have the 38kHz frequency in it anymore. Just the pulses of different length, that encode the data, meaning

The Arduino then reads the length of the pulses (that only consists of sequences of 0V and 5V) and decodes them into a number. This number represents the pressed button.

A proper library for the Arduino can read the different data protocols used in common IR remotes.