1) voltage levels
2) pinout (if it's standarized. I'm no sure central cable is sensor output, my fan has a central black wire and yellor and red on the sides... I'd bet red=+12VDC, and black=0VDC)
Yes, usually sensor wire is yellow, but better check it with meter
It is open collector output from internal magnetic HALL sensors, giving 1 or more negative pulses per revolution.
Apologies, if it;s obvious. But usually, the fans have a part number and manufacturer written on the rotor. You can look up the datasheet for the fan and find everything about the sensor output in it.