Its just a (multidimensional) grid where each point has a vector. A particle moving through such a field will be affected based on its location in that field. Of course its dead simple to implement that in Grasshopper. But of course ,you need to add time and this when it becomes tricky for GH. You could hack it together with Anemone or Kangaroo(?), but its way easier just to scripting that.
But as I said, the magnetic flowlines you see in native GH are basically just the same. It traces particles moving through a vector field and returns you the flowlines. As I said, the Flow plugin is a more generic version of it. But if you looking for a particle simulation with nice animations you need to script this or you need to use another tool.
As that guy said, the creation of the actual geometry was just based on the traced flowlines. So its more an fluid effect. Years ago I did something similar just using Particle Emitter in Cinema4d. You don’t need to reimplent the wheel just to get fluid flowlines. But you should not pretend to give it a meaning.
If you don’t know how to do all this, simply draw these flowlines to make them look alike. This ends up being the same.