The manapua is a sandwich that bakes or steams its own container around the filling, and the container is engineered to be soft on every surface rather than crisp on any. A sweet, enriched white dough is wrapped and sealed completely around a portion of sticky red char siu pork, with no exposed seam and no second slice of anything. The bread is not a carrier the filling is placed onto; it is a closed shell formed to the cargo, and its defining quality is a pillowy, faintly sweet crumb that yields against the savory, lacquered pork inside it. That contrast between a sweet soft dome and a glossy salty center is the whole sandwich.
The craft is in the dough and the seal. The dough is leavened soft and sweet, scaled larger and richer than its Cantonese ancestor so the bread itself reads as part of the meal rather than a thin wrapper, and it has to be strong enough to hold a wet, sticky filling without splitting at the base. The char siu is cooked and chopped before it goes in, glazed sticky and red, because the bake or steam time is set by the dough and not the meat. The closure is structural: the dough is gathered and pinched shut so the filling stays sealed and the heat stays in, and a weak crimp leaks and the pocket fails. The steamed version sets a white, tender, slightly chewy skin, while the baked version takes on a thin golden top, and the choice between them is a choice about how much resistance the shell offers against the soft pork.
The variations are mostly a swap of the filling inside the same sealed sweet dough. Curry chicken, sweet potato, and other savory fillings stand in for the char siu while keeping the dome and the crimp; a baked version trades the steamed skin for a glazed crust. The wider family of stuffed pockets built for portability, the runza, the bierock, the pasty, makes the same structural decision with a sturdier savory dough and reads as its own tradition. Each of those deserves its own article rather than being crowded in here.