it's from the ATMEGA documentation, you need to look at the pin configuration (page 2) and how Output Compare registers are mapped to the processor pins.
here for example I highlighted OC0, 0C1, 0C2 (only A and B for those so 2 pins) and OC5 (A,B and C so 3 pins)
Then you look at how Arduino decided to map the physical pins to the Arduino pins
and that gives you the mapping above
the datasheet explains how PWM works.