John, All purpose flour is white wheat flour milled without the outer bran kernel included. Self rising flour has salt and baking powder included after milling. Same for plain or self rising (wheat)flour. It's just plain white, fine milled wheat flour.

Buttermilk is actually what is left after making butter out of whole sweet cream but in the US you can buy "buttermilk" in the dairy section of the market. Or, you can buy "buttermilk powder" to which you add water to make "buttermilk".