I've never looked up a recipe for either. Even though it's generally a southern dish, both items are so common yankees learn them by helping Grandma in the kitchen. I bet the biscuit recipe is on the Bisquick box, but doing it from scratch is something I've seen on FoodTV more times than I can count.
Quarter kilo breakfast sausage. Garlic or onion optional. Stir in frying pan until little pink remains. Add flour slurry and cook until the flour just barely starts to brown making a light roux and completing the cook of the sausage. It's okay to use a splash of whiskey in the slurry to make it fancy for dinner rather than plain for breakfast. Make richer with cream or thinner with milk. Simmer while the biscuits are in the oven.
I have no idea what the spice mixture is for American breakfast sausage other than sage being an optional extra. It doesn't have fennel seeds like Italian sausage or peppers like chorizo. The fat is about right for the biscuits so most people use the minimum sausage and don't drain - It's a poverty strategy to stretch out every bit of the meat.
MOSTLY how I make it. Cook the sausage as you say, to make a decent family size batch I use 2lbs of bulk sausage, 1lb Sage, 1lb Hot, Jimmy Dean is a National US Brand that can be used, I prefer a local PA Dutch Brand. Here is where we differ, Once the sausage is almost brown I add just enough flour to soak up the delicious pork fat rendered from the sausage and cook stirring until it just starts to turn from white to light beige in color. Then I add Whole Milk, a little at a time until the consistency is "just right" where if you pick it up with a large serving spoon it will drop off with little effort. Then season with salt (amount dependent on how much salt was in the sausage,) and plenty of fresh ground black pepper. I don't like using the flour slurry as I feel the flour does not get sufficient heat that way to get rid of the "Raw flour" taste.
I am not a baker, so my Mrs makes the buttermilk biscuits, and I am not privy to the closely guarded recipe she was taught by a friend from church when we lived in NC. I suspect there is a penalty associated with revealing that recipe much more severe than having the penalty of all our Masonic Obligations inflicted on us!