How is white pizza sauce made?
White pizza sauce isn't nearly as defined as red sauce.  It's largely whatever you want it to be.

It's literally any sauce you top a pizza with that is white.  It is often dairy based (cream or cheese added), but it could also be a thin parsnip puree.  It may contain herbs, it may contain butter, its up to you.  Again, thickness is up to you and whatever works for your dish.