We know what causes magnetic fields and we know what their effects are, but what actually are they? What does it mean for them to be composed of “lines of force”? What is the mechanism of that force? What is actually going on in a magnetic field that the space outside of a magnetic field lacks?
short non-answer is that magnetism comes up when you consider relativistic effects of electrostatics https://phys.libretexts.org/Bookshelves/Relativity/Book%3A_Special_Relativity_(Crowell)/10%3A_Electromagnetism/10.01%3A_Relativity_Requires_Magnetism
i don’t really know how to explain it better
That link is broken
done