Magnetizable objects become magnetized when under the influence of opposing magnetic fields since a winding influences a screw at high rates the induction of a field through upon it is reversed this is the resistance of the metal to release and gain magnetism, permeability.
The motor only needs iron in its winding in the stator if brushless and rotor (armature) if brushed.

Bigger rotors yields less copper in the winding however to get any more in detail I’d have to write a novel.