The whole sandwich rests on one technical fact about scrapple: it is a soft, cornmeal-bound pork loaf that holds together only if it is sliced and fried correctly, and a scrapple sandwich is really a frying problem with bread attached. Scrapple is pork scraps and broth set with cornmeal into a sliceable block. Cut a slab, fry it hot in its own fat until the outside sets into a hard, crackling crust, and the interior stays soft and almost custardy. Put that slab on toast or a roll and the sandwich is done. The bread is incidental; the entire character lives in the contrast between a shattering fried crust and a yielding inside, and getting that contrast is the only craft the sandwich asks for.
It works because everything in the build serves that fragile crust. Scrapple flipped too early or fried too cool never sets and collapses into mush on the bread, so it is left undisturbed on a hot, fatted griddle until a real crust forms before it is moved at all. Toast or a sturdy roll is chosen specifically because soft fresh bread would go to paste against a hot, fat-rich slab, where toast stays firm enough to brace it. The sandwich is usually served stripped down, sometimes nothing but scrapple and bread, sometimes a fried egg and a thin line of ketchup or apple butter as a sweet-acid counter to the savory pork, and that restraint is honest: this is a breakfast-counter and diner food in Delaware and the surrounding mid-Atlantic, built to be cheap, hot, and quick. The Philadelphia version is the close cousin it is most often compared to, the same fried slab on the same kind of bread a state away.
The variations are small and regional. The plain build keeps it to scrapple on toast. The breakfast build adds a fried egg and cheese on a roll. Each of those deserves its own article rather than being crowded in here.