How to determine PWM Frequency of a MOSFET to drive a BLDC motor?
The frequency used for PWM to drive a motor is a design choice.

Choosing a very low frequency can result in:


the motor making audible sounds
the motor might not run very smoothly


Choosing a very high frequency can result in:


the induction of the motor coils preventing the motor from working
more switching losses in the motor driver circuit, it might become hotter which is bad.


So selecting the right frequency is a choice that is done by the system designer. It is a compromise between everything listed above and sometimes other factors as well.

how does the MOSFET circuitry ...100 A in Motor Phases?

It does not produce the current. The current that flows is also a result of the total design. Only if the supply can deliver at least 100 A, the MOSFETs are capable of conducting 100 A, the MOSFETs are cooled well enough to support 100 A, the circuit board supports 100 A, all series resistances are low enough to support the flowing of 100 A and finally the motor is designed to draw up to 100 A, will 100 A flow. That 100 A will not just flow by itself, a lot of conditions need to be satisfied to make such a large current flow in a motor driver.