The smash burger is defined by a single violent technique: a loose ball of beef is pressed hard and flat onto a screaming-hot griddle so that maximum surface meets maximum heat for a maximum browned crust. The patty is not shaped and then cooked; the cooking is the shaping. That forced contact is the entire sandwich. It drives a deep, lacquered Maillard crust across the whole face of a thin patty, which is a different goal from every thick-patty burger that protects a juicy interior, because here the crust is the point and the thinness is what makes the crust possible.
The craft is in the press, the timing, and the surface. The flat-top has to be genuinely hot before the ball goes down, and the smash happens once, immediately, with firm even pressure, then the patty is left alone, because pressing again after the crust forms only squeezes out juice and gains nothing. It is held until the edges go dark and crisp and the meat releases on its own, then flipped once; the cheese goes on while the patty is still on the steel so it melts into the crust rather than onto it. The patty is thin by design so the entire thing is crust and edge with almost no soft interior, which is why the build often runs two thin patties rather than one thick one: doubling restores some heft without sacrificing the ratio of seared surface to meat. The bun is soft and lightly toasted so it compresses to the patty and absorbs fat without fighting it. This is a cook engineered for crust at speed on a hot surface, which is exactly why it scales on a griddle line.
The variations stay inside the thin-pressed maximal-crust idea and mostly change what is smashed into or onto it. The Oklahoma onion build presses shaved onion into the patty so it fries into the crust under the cheese; doubles and triples stack the thin patties; bacon and chile versions add to the same architecture. It is one regional dialect of the American burger, distinct from the small steam-onion slider and from the cheese-stuffed and steamed-cheeseburger builds. Those deserve their own articles rather than being crowded in here.