The fish taco is a frying problem wrapped in a folded bread. A mild white fish is dipped in a beer batter and fried hot, then laid in a warm corn or flour tortilla under a heap of shredded cabbage and a thin crema. The tortilla is the structure, not a plate: a flexible round folded around a complete, wet, hot filling so it can be eaten in the hand. Everything in the build is arranged to protect a crisp batter inside an open, soft, steaming wrapper, which is the central tension of the form.
As an open handheld it works because each component defends the fry. The batter is thin and beer-leavened so it sets into a light, blistered shell rather than a heavy crust, and it has to stay crisp for the length of the taco even though it is sitting against a warm tortilla. The cabbage is the structural counter: shredded raw, it brings a cold crunch and a dry bulk that the soft tortilla and the wet fish both lack, and it physically separates the crema from the batter so the coating does not go soggy before the second bite. The crema, often thinned and brightened with lime, is applied as a stripe rather than a flood so it adds acid and fat without soaking the shell. The tortilla is doubled or warmed pliable so it folds without cracking under a fragile filling, and the whole thing is built to order and eaten fast, because the gap between fryer and hand is where this sandwich is won or lost.
The fish taco belongs to the American taco, burrito, and wrap family, the immigration map of flexible-bread sandwiches, and carries the Baja California idiom into San Diego. Its close relations split on the catch and the cook: the grilled or blackened fish version that drops the batter entirely, the shrimp taco that runs the same slaw-and-crema frame, and the Ensenada-style build with a creamier white sauce and a fattier batter. Each of those deserves its own article rather than being crowded in here.