The Detroit coney is decided at the moment a ladle of sauce is laid down the length of the dog, and that motion describes the sandwich better than any ingredient list. A natural-casing beef frankfurter goes into a steamed bun, a stripe of yellow mustard runs under or over it, a loose beef chili is spooned along its top, and a heavy scatter of finely diced raw white onion finishes it. The defining component is that chili, and the Detroit version of it is built wetter and smoother than its inland cousins: a fine-ground, beanless sauce loosened until it spreads and soaks rather than sits, closer to a thick gravy than a stew. It is engineered to bleed into the bun and cling to the casing along the whole dog, which is why a Detroit coney is eaten fast and not picked up casually.
The craft is in the texture of that sauce and the order it goes down. Ground fine and cooked with enough rendered fat and liquid to stay pourable off a ladle, the chili coats the frankfurter instead of falling off it in lumps, and that is the entire reason the format survives the trip from steam table to hand. The frankfurter is the firm, salty, snapping spine the soft wet load needs; without the natural casing the sandwich has no texture at its center. The bun is steamed limp on purpose so it yields against the dog and absorbs the sauce that escapes rather than crusting and shedding it. The raw onion is not garnish but the cold sharp counter that cuts the rendered fat, and the mustard is the acid that keeps a rich savory pile from reading flat. Held hot and dressed in seconds across a crowded downtown counter, it is built to be made by the hundred and handed over without coming apart, eaten standing or at the counter before the bun gives way.
The variations run along the same loose-sauce idea with the spicing and consistency argued city by city. The drier, almost ground-beef Flint sauce sits on top rather than soaking in; the wetter inland chili-dog builds push the sauce further toward soup; the Cincinnati line buries a sweet-spiced chili under a haystack of fine cheddar. Each is a codified build with its own town defending it, and each deserves its own article rather than being crowded in here.