hi
I watched this video , I have question why he is change the the angle 90 degree for what?

vector cylindrical




at 4.43

 

good question. Think about the direction tha phi (the angle) travels as it increases. It continually points in the direction of rotation. It always points tangentially to the circle that phi sweeps as it increases from zero to 2*pi radians.
This is similar to the zed unit vector, which also points in the same direction that zed increases as it goes from zero to infinity.
Hope that helps.