The bacon biscuit is defined not by the bacon but by the biscuit, which is doing a job no other breakfast carrier does. A buttermilk biscuit is tender, fatty, and faintly tangy, built from cold fat cut into flour so it bakes up in flaky layers rather than a tight crumb. That structure is a deliberate counter to a hard, salty strip of bacon: the bread brings the richness and the give, the bacon brings the salt and the snap, and the contrast is the sandwich. Put the same bacon on toast and it is a different, lesser thing.
The craft is in the biscuit and the timing. The biscuit has to be split warm, while the interior is still soft and the layers still pull apart, because a cold biscuit goes dense and stops being the yielding half of the contrast. The bacon is cooked crisp and laid flat so it lies in the split without sliding, and the biscuit's fat does the work that a sauce would do elsewhere, lubricating each bite so the dry crumb does not read as dry. An egg is the common addition: cooked soft and folded to the footprint of the biscuit, it binds the bacon to the bread and turns a two-part sandwich into a single mass that survives a commute. The order matters, with the egg and any cheese against the warm cut faces so the heat sets them before the biscuit cools.
The variations track the Southern breakfast counter and stay close to the same frame. Sausage stands in for bacon as a softer, sage-seasoned patty; a thin smear of sawmill gravy turns the biscuit into something between a sandwich and a plate; country ham brings a saltier, drier cure that the biscuit's fat is there to balance. Each of those is a codified build with its own logic and deserves its own article rather than being crowded in here, since the biscuit is generous enough to anchor a whole shelf of breakfast sandwiches.