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?
So, the purveyor of the electromagnetic force is the photon, as in visible light and radiation. If you look into it you find talk of “virtual photons” being the purveyors of magnetic force, but it seems they are more of a mathematical construct, and if you read enough about it, it becomes clear that it’s a whole lot of technical language that boils down to, “we dunno”.