The Friday fish fry sandwich is a Wisconsin tradition before it is a sandwich, and the batter is where the tradition lives. The fillet, cod or lake perch, is dropped in a beer batter rather than a dry breading, and that batter fries into a thick, blistered, slightly bitter shell that puffs away from the fish instead of clinging to it. That airy crust is the identity. It is the reason the sandwich stands as its own thing rather than a fish sandwich that happens to land on a Friday: the beer in the batter does structural work, lightening the coat so it stays crisp and shatters on the bite rather than turning to a dense fried crust.
The craft is a frying problem managed against a soft bun. The fillet has to be thick enough to stay moist inside that heavy batter but cooked fast and hot so the shell sets before the fish steams itself flabby. The bun is plain and soft on purpose, a kaiser or a brat roll, chosen because a crusty roll would fight a delicate battered fillet and win. Tartar sauce is not garnish but the working counter, supplying the acid and fat the lean fish lacks, and it goes on the bun rather than over the crust so the coating stays crisp to the last bite. The classic plating keeps it close to the supper-club standard: the fillet, the sauce, often a slice of raw onion and a wedge of lemon, served fast because a battered fish that sits goes soft from its own steam. Eat it the minute it lands.
The variations follow the lake and the kitchen rather than the recipe. Perch runs sweeter and more delicate than cod and makes a lighter sandwich; bluegill and walleye stand in where the boat brought them; some supper clubs swap the bun entirely for rye and lean on a horseradish sauce. The baked or broiled version for the Lenten table drops the batter and becomes a different sandwich. Each of those is its own regional reading and deserves its own article rather than being crowded in here.