The crispy fish sandwich is a sandwich engineered to keep a fried crust crisp inside a closed, soft, sauced build that is actively working against it. A mild white fillet, often pollock or cod, is beer-battered or breaded and fried hot, then shut into a steamed bun with a cold tartar sauce and frequently a half-slice of cheese. Every other component is arranged to defend that coating: the trip from fryer to hand is short, the sauce insulates rather than soaks, and the bun is chosen to be too soft to fight back. The crust is the entire sandwich, and the rest is its protection.
The craft is in the fish and the timing. The fillet has to be thick enough to stay moist and flaking inside its shell but even enough to cook through before the coating darkens past golden, which is why a uniform rectangular cut is standard rather than a tapering natural fillet. The batter or breading is built to fry into a rigid, bubbled crust that holds its texture for the few minutes between the fryer and the first bite; held too long, it steams from the fish's own heat and goes limp, which is the failure this whole assembly is designed to delay. Tartar supplies the acid and fat the lean fish lacks, and shredded lettuce or a pickle adds the cold crunch it has none of. The bun is deliberately pillowy and lightly steamed so it compresses to the fillet instead of competing; a crusty roll would dominate a delicate piece of fish and win. The fast-food and Friday-fish-fry reality is a fryer dedicated to it, fillets dropped on the ticket, the sandwich wrapped warm so the crust survives the counter.
The variations are mostly a coating argument. The beer-battered build leans on a thick, craggy, almost tempura shell; the breadcrumb build runs a finer, sandier crust; the spicy version works heat into the dredge or the sauce. These are small turns on a fixed structural problem, and the wider American fish shelf, the regional grouper and walleye and lake perch, the griddled crab cake, the Southern fried catfish, solves the same problem with different water and deserves its own articles rather than being crowded in here.