So many valves...

the wastegate physically controls the boost pressure by allowing excess exhaust gas to bypass the turbo resulting in less boost pressure, and it in turn is controlled by the n75 valve - this is in effect an electronic solenoid, which in turn controls the behaviour of the wastegate, and gives the ecu the electronic control of the wastegete.

the wastegate has a weak spring and without the n75 valve a low turbo boost of usually 3 or 4 psi will be mechanically maintained by the wastegate leaking excess exhaust gases past the turbo.

the n75 valve fools the wastegate into thinking the pressure is lower than it really is, by diverting some of the existing boost pressure away from the wastegate and back into the inlet system - the wastegate therefore sees a lower boost pressure than really exists and stays shut, thus allowing a much higher boost pressure to be generated before the wastegate maintains boost pressure.

different types of n75 valve have different ranges of action, and so for any given range of electronic signal from the ecu result in more or less boost, or more or less agressive rise in boost, all depending on the behaviour of that particular n75 valve

changing n75 valve type will change either the ultimate boost, the way that the boost rises, the overboost response to full throttle or any combination of these things.