What causes magnetic fields to occur?

A magnetic field is created when an electric charge moves. This can happen in a few different ways.

Each electron is a tiny magnet because it is an electric charge that is spinning. Electrons form shells, and when a shell is full, all of the electrons are paired up, with each pair's members pointing in opposite directions, resulting in the absence of a net magnetic field. Elements that have unpaired electrons that can be aligned to form permanent magnets include iron, cobalt, and nickel.

Strong electric currents passing through a coil of wire are what create an electromagnet; an electric current moves charges, which creates a magnetic field when it flows.

Strong magnetic fields are produced by charged particle plasmas flowing as convection currents within the Sun.

Electric charges in motion are the cause of magnetic fields.