How is white pizza sauce made? As far as I know, the only difference between white pizza and red pizza is the sauce. 

I know that the white sauce is dairy-based, but what is it exactly? What are the main ingredients, is it thickened somehow? Is there something special to pay attention to when making white sauce? How thick should it be exactly? Should it contain herbs or not? Oil? How does it combine with the cheese, is the cheese supposed to melt "into" it?
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.