The doorstop is defined by the bread, not the filling, and specifically by how thick the bread is cut. It is named for the slab it resembles: bread cut deliberately, almost defiantly, thick, far thicker than a machine-sliced loaf would give, hand-sawn from a fresh loaf so each slice has real heft. Whatever goes inside is secondary to that decision. The doorstop is a statement that a sandwich can be generous to the point of excess, that the bread itself is part of the meal rather than just a wrapper for it, and the thickness is the design rather than a clumsy cut.
The craft is structural before anything else. A thick slice has to come from a loaf with enough crumb structure to hold its shape under a heavy fill, so the bread is usually a sturdy white or a bloomer rather than a soft sandwich loaf that would compress to nothing under the weight. Butter goes on heavily and to the edges, working as the flavour bridge and the seal that stops a substantial, often greasy filling soaking two thick faces of bread into stodge before it is eaten. The fill is generous to match the bread, because a mean filling between two slabs reads as mostly bread and the proportion is the whole point. It is pressed down so a tall, loose build compacts into something a single hand can grip and bite without it falling apart.
The variations are a question of what fills the slabs rather than how they are cut. Cheese and pickle with thick Cheddar against Branston; bacon and egg for a builder's-style breakfast; cold roast and stuffing folding a Sunday dinner into the doorstop slices. The builder's sandwich is the same thick-cut idea named for its customer rather than its shape, and each of these fillings deserves its own article rather than being crowded in here.