A hamburger is decided by what happens to the surface of the patty, not by what is piled on it. Ground beef has no structure of its own and no developed flavor until heat builds a crust on it: a hard sear on a flat-top or grill is the entire reason the sandwich tastes like anything. The patty is the only fixed point. Beef, ground and formed loose enough to stay tender, seared hard, set on a soft bun with lettuce, tomato, raw onion, pickle, and a condiment or two. Those toppings are a cool, wet, acidic frame, and the frame exists to make the seared, fatty center read as balanced rather than heavy. Get the crust wrong and no amount of dressing rescues it.
It survives as a sandwich because the bun is engineered to fail gracefully. A beef patty sheds fat and juice from the moment it leaves the heat, and a plain, slightly sweet, pillowy bun is chosen precisely because it absorbs that bleed and compresses to the meat instead of resisting it. A crusty roll would fight the patty and win, shredding the structure; the soft bun yields with it. Many builds toast the cut faces of the bun on the flat-top first, a structural move as much as a flavor one: a thin seared layer on the crumb slows the juice from reaching the outer shell so the bottom holds for a few more bites. The tomato is the weak point in the build, since a wet slice floods the bottom bun, so it is salted, set against the lettuce, and kept off the hot surface. The lettuce and raw onion are the crunch the patty does not have, and the lettuce often sits directly against the bun as a second moisture shield. The order of assembly is itself a decision: condiment placement, whether the lettuce shields the bun from the tomato, how thick the patty is cut against how much juice the bun can take before the last bite.
The variations are mostly geographic and each deserves its own article rather than being crowded in here. Add a slice of cheese melted onto the patty on the heat and it becomes a cheeseburger, a single swap that earned its own name. The smashed style maximizes crust on a thin patty; the thick steakhouse patty argues the opposite case for a juicy interior; the slider shrinks the whole thing to a few bites with griddled onion. The regional builds, the green chile cheeseburger, the butter burger, the patty melt's cousin on rye, the bison and cheddar versions, change the center or the carrier and keep the architecture. The non-beef builds, turkey, salmon, bean, and the lab-grown patties, hold the form and replace the patty entirely. The chain icons standardized particular assemblies and shipped them everywhere, which is its own kind of regional accent.