In my settings, the Dragonborn resembles a Medium size Dragon. It is a "Humanoid Dragon", both of the creature types are equally true and active.
As with all dragons, they are biologically a kind of snake. The mind of these draconic snakes distinguish from other snakes, analogous to how humans do from other apes. Both humans and dragons evolve from a Beast.
By definition the dragon is an amalgam of a snake with certain features resembling those of other animals. It is an evolution that involves shapeshifting magic, and the non-snake features can come from imitation or from magical genesplicing. Which other animals combine into the snake amalgam depend on regional archetypes. The anatomical forms of dragons vary by indivual, and trend by cultures.
Norse Drekar
The anatomical form of a Norse dragon is typically a horned snake with arms. The face has leonine features, with fangs. The arms resemble eagle legs, with prehensile handlike eagle claws. The snake body is that of a european adder, with patterns of scales of glossy black with metallic silver andor gold. Females tend to have a brassy sheen, sometimes to a degree that appears fully solid brass. The horns are a goatlike V-shape sweeping back from between and behind the eyes. The Drekar locomote by slithering like a serpent, while using their arms for hands, but can use their forearms to assist locomotion. They typically lack wings, but known individuals develop eagle wings, as adults or ancients. They are born as if natural adders, but soon the V-pattern of the scales of the back of head the develop into actual horns. Eventually arms form at puberty. Some adult Drekar develop wings. The anatomical form never develops hindlegs, and always slithers. Each anatomical transformation occurs during a skin shedding.
The draconic anatomical form of the Drekar is also known as the linnormar form (cognate with English lindworm and German lindwurm), where linn relates to the "constricter" snakes that grapple prey. Most dragon species, including those of Metallic and Chromatic, include individuals with a linnormar form. Most dragons of the Nordic regions develop this anatomical form. The D&D elemental Salamander resembles a linnormar form, albeit the Drekar retain a more fully serpentine body, including snakelike neck and head.
Likewise some Dragonborn indivuals develop as linnormar, slithering without hindlegs, especially in Nordic regions. Some become Large or larger when accumulating magical power while advancing in levels. Some form wings at a higher tier during a shedding.
British Dragon
The British draconic anatomy is typical for the Chromatic species, where the snake exhibits features of wolf and bat, rather than lion and eagle. Normally the British anatomy develops six limbs: forelegs, hindlegs, and bat wings. Their bodies remain serpentine. The 2024 Dragons mainly correlate with the British draconic form. Nevertheless, the anatomy of individual Chromatics and Metallics can vary significantly.
Asian Dragon
The Asian draconic anatomical form, such as Chinese, Japanese, and Korean, are a complex snake amalgam, including features of fish and camel, with branching deer horns, and other animals are possible. Typically these have four limbs: forelegs and hindlegs, and slither bouyantly thru the air. The overall impression of a snake is typical.
Babylonian Dragon
These dragons can display many anatomical forms, but are mostly some amalgam of snake with lion and eagle. One Mushkhushshu form features the head of a horned viper (sometimes the horn forming vertically upward from above each eye, instead sweeps back then curls at the end. The skin is snake scales. The body is lithe but lionlike, with lionlike forelegs and eaglelike hindlegs. The tail resembles that of a lion with a tuft of fur at the end, but can undulate like snake.
Quetzalcoatl Dragon
The Quentzal-coatl Dragon form ("feathered serpent"), typically resembles a snake with wings. They are typical for Mesoamerican regions, but also occur elsewhere around the world. Any Dragon species can have individuals developing the feathered serpent anatomical form. The wings are feathery and sometimes the back or all of the serpentine body is too. Some individuals lack any limbs but are partially or fully feathery.
(Note, the "Quetzalcoatlus", with the Latin -us suffix, refers to a genus of pterosaurus species. This birdlike dinosaur is Huge with body and neck resembling a giraffe, and ambulating similarly on all fours, but with very long thick beak and foldable wings extending from the forelimbs. It takes flight by jumping upward, with powerful forelimbs, then flapping down with its wings extending. The Quetalcoatlus is the largest natural flying animal.)
Wyvren Dragon
The Wyvren anatomical form is birdlike with wings and hindlegs, locomoting by walking and running. The tail tends to be long and serpentine, sometimes with a barbed end, but can coil up out of the way to balance while ambulating.
3e Dragon
The D&D 3e tends to be less serpentine, with a more stocky lionlike body, or wolflike body. The neck and tail tend to remain long. Overall, the impression is more like quadrupedal dinosaur, albeit the forelegs can be used prehensilely like hands.
Any Dragon individual, including any Dragonborn individual, might develop any of the above draconic anatomical forms. Under the influence of the humanlike characteristics, some Dragonborn lack tails.
There are also dragonlike biological relatives that are a nonsapient Beast, referred to as a "Drake". They are natural snakes but develop under the influence of draconic magic. Individual Drakes might also exhibit any of the above anatomical forms. Korean dragons often originate from imugi indivuals, gargantuan snakes who aspire to become dragons.