The best biscuits are made with White Lilly flour, and buttermilk, which causes the baking powder to activate and produce fluffy biscuits. Normal milk will not do this. Before adding the buttermilk, you need to mix the flour, shortening, butter, and baking powder cold and dry, and there is a special kitchen tool for this.
...a special kitchen tool...
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...or just grab 2 dinner forks.
Normal milk does fine with baking powder, which is what’s in self rising flour. It’s baking soda that requires the acid of buttermilk. You can certainly use buttermilk with the baking powder instead of baking soda, but you must have something acidic to activate baking soda.