Undervolting aims to keep the same performance at a lower power draw. But because modern SOCs scale their frequency dynamically based on temperature, a lower voltage means slightly better performance
Yes. Essentially, every chip coming out of the fab will be slightly different, so you need to find a voltage curve that works for all of them. Some will be able to work with - 50mv, some -15, some won't work at all unless they're right at that factory defined voltage.
Think of amps and volts like a hose and water. If you put your thumb over the hose the water shoots out faster. Undervolting works in a similar way. So the goal is to lower the volts but maintain or even increase the amps to improve performance.