how are sunspots formed?

Normally magnetic fields follow smooth paths from one magnetic pole to the other. It's almost as if there's some sort of massive network of pipes carrying the magnetic field from one part of the Sun to another. This is what we imagine as a magnetic tube.

When the magnetic field bursts through the surface like that, it can slow down the flow of energy from the inside of the Sun - that's what makes the sunspots cooler than the surrounding photosphere.

Sometimes, a magnetic tube bursts out of the Sun's surface and then dives back down again, as in this diagram. This creates a pair of magnetically positive and negative sunspots, like the poles of a magnet.