The disco fries sandwich takes a side dish and treats it as a filling, which is the whole conceptual move. French fries, brown gravy, and melted cheese are the entire interior, set on bread that has become a floor rather than a wrapper. There is no meat doing the structural and flavor work a sandwich usually leans on; the fries are the substance, the gravy is the binder, and the cheese is the seal. It is a New Jersey diner build that decides a plate of loaded fries is close enough to a sandwich to put bread under it and call it one, and the engineering question that follows is how to keep that floor from dissolving.
The craft is in heat and timing on a base that wants to fail. The bread is griddled or toasted firm so it can take a flood of hot gravy without turning to mush before the plate is finished, the same problem every open-face hot-plate sandwich has to solve. The fries go down still crisp, because they will soften under the gravy quickly and the only window where the texture works is the first few minutes. Brown gravy is ladled over while everything is hot so it binds the loose fries into a mass the fork can lift, and the cheese, typically melted mozzarella or a cheese sauce, is added so it sets into a layer that holds the pile together rather than a separate topping sliding off. This is late-night and post-bar diner food, assembled fast and meant to be eaten immediately with a fork, because served slow the base is gone and it collapses into wet bread and cold starch.
The variations stay inside the gravy-and-cheese-over-starch logic and are mostly regional dialects of the same idea. The cheese-fries build skips the bread and the gravy; the poutine line runs curds and a thinner gravy without the slice underneath; the broader open-face family spreads into the Hot Brown, the horseshoe, and the slinger, each burying its own filling under sauce on a single piece of bread. Each of those is a codified build with its own following and deserves its own article rather than being crowded in here.