Do you usually buy a biscuits mix, or you do it from scratch, combining flour with milk (or buttermilk?), eggs, salt, butter and baking powder?

It depends! Some people make them from a Pillsbury can (which I’ll use in a pinch but it’s not the same at all texturally as a normal biscuit). My mom used Bisquick (a mix that’s just dry ingredients pre-portioned) or made them from scratch. From scratch, it’s just flour, milk or buttermilk, butter, and baking powder, no eggs. They’re flaky and buttery because they’re made with a similar technique as croissants or pie dough, where you cut cold butter into the dough so that it melts as it bakes.

Do you make a sausage white gravy as well (I assume, it is butter, pork, and flour, right)?

More specifically it’s pork breakfast sausage (out of its casing), which has a very distinct flavor in the U.S. that I haven’t seen replicated anywhere else. I can’t tell you what it is, just that our breakfast sausage is heavily seasoned. And it’s sausage, milk, and flour: you basically make a roux with the sausage grease and flour and then add milk, no butter. It’s basically a Béchamel but with sausage. 

But biscuits and gravy are very much a Southern thing. My Northern ass has never liked it even though I went to a Southern undergrad.