Question about the resulting real voltage of pwm (pulse-width modulation)
No, not in general:

If you measure fast enough, you'll see the high and lows.

Now, if you add a low-pass filter after the PWM unit, you will indeed filter out the changes, and "smoothen" the PWM to an average value.

So, whenever "I read somewhere that one can simply...", it's always worth asking which simplifications were made.