BLDC phase voltage wave form
I want to understand why there is a spike in the back EMF as shown in the figure below. As described in theory the Back EMF of floating phase ant any instant should ramp up or down but why there is a huge spike? How to reduce the spike?

It is demagnetization of the phase winding, as low side fet is off and high side diode is conducting.

My second question is at lower rpm why there are so much noise in back EMF? Is it actually happening to the back EMF or it is just an oscilloscope trigger visualisation.?

It seems to be the ESC modulating the low side fet instead of high side. When you reverse the waveform, it will be a clean signal at GND level, and PWM signal at VBUS level.