The burnt ends sandwich is built on the single fattiest, most worked part of the brisket, and that is its whole identity. Burnt ends come from the point, the thicker, heavily marbled muscle of a packer brisket. The point is smoked with the rest of the brisket, then separated, cubed, and returned to the smoker so the surfaces render and bark a second time, often after a turn in sauce. The result is cubes that are caramelized and chewy on the outside and silky with rendered fat on the inside. This is not pulled or sliced meat asked to stretch over a bun; it is the densest, richest possible payload, and every other decision in the sandwich answers to that.
The craft happened across two trips through the smoker, and the sandwich only has to keep the cubes from overwhelming themselves. The double smoke is the point: the first cook renders the point to tenderness, the second concentrates the bark and firms the cube so it holds its shape in the hand instead of falling to mush. A plain, soft, slightly sweet bun is chosen so that it disappears under the meat and soaks up the rendered fat and the glaze. The glaze, a thick, sweet, often tomato-based Kansas City sauce, is part of the structure rather than a condiment: it lacquers the cubes, ties them together so they do not scatter out of the bun, and supplies the sweetness that balances the heavy smoke and fat. Pickles or a sharp slaw are the standard counter, the acid and crunch a pile of rendered, glazed beef has none of, and the only thing keeping the sandwich from reading as relentless.
The variations are mostly about what gets cubed and how it is dressed. A pork belly version applies the same twice-cooked, cube-and-glaze method to a different fat; the level of sauce and the addition of slaw move it richer or sharper. It belongs to the same family as the pulled pork sandwich and the rest of American barbecue, where the smoker does the work and the sandwich just carries it, and those relatives deserve their own articles rather than being crowded in here.