The bagel is a ring of yeasted dough boiled and then baked, and as a sandwich it is defined less by what goes inside than by what the cooking does to the bread. The angle is the boil. Dropping the shaped ring into water before the oven sets the crust and gelatinizes the surface starch, which is what gives a true bagel its glossy, chewy skin and dense, close crumb. That structure is the whole point: a bagel sandwich is built on a bread engineered to stay sturdy and chewy under a wet, heavy filling rather than to be soft and yielding like a roll.
The build starts with the ring itself and what it can take. A well-made bagel has a taut, slightly leathery exterior, a tight interior with no large holes, and enough body that it can be split through the equator and hold a substantial load without tearing or going soggy. It is usually sliced and lightly toasted so the cut faces firm up and resist moisture, then dressed. The classic load is a spread that clings, soft cheese or a similar base, plus something briny or sharp and something fresh for contrast, but the bread's job stays constant regardless of filling: a sturdy, chewy frame that survives the spread. Done right, the bagel has a crust with real resistance, an interior that is dense but not raw or doughy, and a toast level that keeps the structure intact under the topping. Done wrong, it is a soft bread roll shaped like a ring, with no chew, no crust, and a crumb that collapses the moment a wet filling touches it. The seeds or toppings on the outside, sesame, poppy, salt, dried onion or garlic, add aroma and crunch but do not change that underlying logic.
It varies first by the dough and the boil: a longer cold ferment and a heavier hand on the boil give more chew and a deeper crust, a shorter, lighter process gives a softer ring closer to a bread roll. From there it varies by topping and by the regional habits of where it is being made, the seed mix, the size of the ring, and the thickness of the crust all shifting by place. Those regional and filled forms are recognizable on their own and deserve their own treatment rather than a footnote here, but they all return to the same idea: a boiled-then-baked ring whose chew and crust are engineered to carry a sandwich rather than only to be eaten plain.