Question

why a moving charged body generates magnetic field?

Answer 1

A charged particle moving without acceleration produces an electric as well as a magnetic field. It producesan electric field because it's a chargeparticle. But when it is at rest, it doesn'tproduce a magnetic field. All of a sudden when it starts moving, it startsproducing a magnetic field

Answer 2

EM Force is one entity and hence can't be treated differently
Take a charged particle:
In its rest frame, it appears to generate an electric field only and no magnetic field at all. From a different frame of reference (in particular one in relative motion):
we'll see the charge moving, thus a current which generates a magnetic field as well.

This does not mean that setting the particle in motion somehow flipped a switch within the particle - rather, it's an artifact of our choice of frame of reference: Observers in relative motion will measure different strengths of electric and magnetic fields the same way they measure different velocities and momenta.