Multipliers! how are they set!

Wusy is right. They call the process binning. Most chips are binned at 90% of thier expected speed. (every once in a while better chips are lower binned, because of demand for that speed) Once a multiplier is decided, the bridges that control multipliers are burned in place, using a laser.
In the case of Amd chips, and some Intel chips, the lower multipliers are also installed, for C&Q.