What does "happen to" mean here? How can I use this expression?
To "happen to" do something means that it is random or by chance. In the case of the video here, "happen to know a thing or two about scary" means that by chance she knows about scary things. You can use the expression "happen to" whenever you are expressing that something was a coincidence or was done by chance and was not known before. Ex: I happened to see my neighbor at the store today. Ex: The dinner she cooked for us happens to be my favorite.