A while back I wrote an uncharacteristically formal thing for the “GIS&T Body of Knowledge” about multivariate maps. You can find it here.
Expanding on that, here are a series of videos that describe, with more visual examples, what they are and how to make them.
Part 1
What the deuce are they? Why would you make them? Should you make something else?
Part 2
Lets roll up the sleeves and try to make a few of these things in ArcGIS Pro.
Part 3
What about on the web?
Happy Multivariate Mapping! John