The Texas brisket sandwich is defined by what is missing from it: sauce. Smoked beef brisket goes onto plain white bread or a soft bun with a few slices of dill pickle and raw onion, and any sauce is set on the side for the eater to add or ignore. The restraint is the whole point. Texas treats the smoke and the bark as the finished flavor, so the sandwich is built to keep them in front and let the bread, the pickle, and the onion stay deliberately quiet around them.
The craft happened in the smoker, hours before the sandwich existed. Brisket is a tough, fatty cut held low over post oak until the connective tissue renders and the meat slices clean without falling to shreds, with a dark peppery bark on the outside and a smoke ring under it. The sandwich's only build decisions are the cut and the carrier. The brisket is sliced against the grain, fatty point or leaner flat depending on the order, thick enough to have presence and tender enough to yield. The bread is chosen to disappear: spongy white sandwich bread or a plain bun that soaks up rendered fat and gives the hands something to hold while the meat does all the talking. Pickle and raw onion are not garnish; they are the sharp, acidic, cold counter that keeps a rich pile of fatty beef from reading as heavy, and they are doing the job sauce does elsewhere.
The variations are the regional barbecue argument with a map. Central Texas runs the meat-market tradition of brisket on butcher paper with white bread, pickle, and onion and no sauce at all. East Texas chops the beef and sauces it freely; the chopped brisket sandwich folds the burnt edges and bark into a looser build. Outside Texas the same cut runs under sweet tomato sauces that Central Texas would treat with suspicion. Each of those is a codified style with its own region defending it, and each deserves its own article rather than being crowded in here.