I have been reading this [current mirror/constant current circuit using MOSFETs](https://www.allaboutcircuits.com/technical-articles/the-basic-mosfet-constant-current-source/).

How does the current not change through Q2 when the voltage at Q2 increases? Ref. Fig1
Where does the excess voltage go when Vcs is increased?
Can someone explain the complete working in simpler terms?