The Chinese roast pork sandwich is defined by a filling that arrives already seasoned, glazed, and finished, so the sandwich barely has to do anything. Char siu is pork that has been marinated in a sweet, savory, faintly fermented mixture and roasted until its edges lacquer into a sticky red crust. By the time it is sliced for a sandwich it is a complete flavor system: sweet, salty, smoky at the char, with a glossy bark on every slice. The sandwich's whole job is to get out of the way of that, which is why the build is so spare and why a Chinatown bakery can sell it for almost nothing.
The craft is in the slicing and the restraint. The pork is cut across the grain into thin slices so each one carries some of the lacquered edge and some of the tender interior, because a thick slab would read as one dense note and bury the glaze that is the point. It is laid on a split roll or a length of soft baguette, the kind a Chinese bakery already bakes for its other buns: a thin, tender crust and an airy crumb chosen to disappear rather than fight a delicate, sticky filling. There is little else by design, sometimes a brush of the roasting juices or hoisin, sometimes a few stems of scallion or cilantro for a sharp green cut against the sweetness, and nothing that would compete with the pork. The restraint is not minimalism for its own sake; it is the recognition that the meat was finished hours ago and the sandwich is a carrier, not a recipe.
The variations stay close to the bakery case. The same char siu goes into a steamed or baked bun rather than a roll, which is a different bread problem with the same filling; a roast duck or soy-sauce chicken version runs the identical spare build with a different glazed protein; a pork belly cut trades lean slices for crackling and fat. Each of those is a codified build with its own logic, and each deserves its own article rather than being crowded in here.