The Buffalo chicken sandwich is defined by an emulsion: cayenne pepper sauce, the Frank's RedHot style, whisked with melted butter until the two become one glossy, clinging liquid. That butter is what separates a Buffalo build from a sandwich that has merely had hot sauce poured on it. The fat rounds the vinegar's edge, carries the cayenne onto every surface, and gives the sauce enough body to coat a fried fillet without immediately running off it. The blue cheese is the obligatory other half of the same idea. Cool, salty, and funky, it exists specifically to stand against the heat, and the sandwich is built around that pairing the way the wing it descends from is.
The craft is in keeping a wet, fatty sauce from defeating a crisp crust. The chicken is brined and fried hot for a craggy coating sturdy enough to survive being tossed in liquid, then it is tossed, not drizzled, so the Buffalo sauce coats it completely the way it coats a wing. The defense against the resulting sogginess is the rest of the build: a soft, pillowy bun that gives way instead of fighting the fillet, a thick swipe of blue cheese dressing that doubles as a moisture barrier between sauce and bread, and crisp lettuce and often a celery-style crunch carried over from the wing plate. The cool, the fat, and the crunch are not optional extras; they are the structural counterweights that keep the hot, vinegary, greasy fillet reading as balanced rather than punishing. Assemble it and serve it quickly, before the sauce works through the crust.
The variations are mostly small swaps on a settled idea. Ranch stands in for blue cheese in the milder reading; a grilled fillet drops the crust and becomes a different, lighter sandwich; the heat is dialed up or down by the cayenne-to-butter ratio. It sits in the same family as the Nashville hot chicken sandwich, a separate regional answer to carrying aggressive heat on a fried fillet, and that relative deserves its own article rather than being crowded in here.