The Texas breakfast taco is defined by the bread it folds into: a soft flour tortilla, not corn, warmed on a comal until it is pliable enough to wrap a hot, loose, scrambled filling and hold it in one hand. That flour tortilla is the engineering. Corn would crack under egg and grease; the flour round flexes around a wet load and seals it, which is the entire reason this works as a handheld meal rather than a plate. Eggs are the binder that everything else hangs on, and the tortilla is the structural decision that makes it a sandwich form at all.
The craft is in matching a flexible carrier to a filling that wants to fall apart. The eggs are scrambled soft and kept loose so they bind the bacon, the potatoes, and the cheese into a single mass rather than a slide of separate parts; cooked hard, they would shed everything out the open end. The potato is doing structural work, absorbing rendered bacon fat and egg moisture so the interior stays dry enough to fold and the tortilla does not go to paste before the second bite. Cheese is added warm so it slumps and glues the fill together. Salsa is the acid and heat that a rich egg-and-pork core needs to not read as one heavy note, and in the Austin and San Antonio idiom it goes inside on assembly or rides alongside, the eater controlling the dose. It is built in seconds on a flat-top and meant to be eaten standing up, on the way somewhere.
The variations are a fixed grammar of fillings ordered by shorthand: bacon, potato, and egg is the baseline; chorizo and egg, bean and cheese, and migas, where fried tortilla chips go into the scramble for crunch, are the codified Texas builds. Each is its own settled order rather than an improvisation. The breakfast taco sits in the broad American taco, burrito, and wrap family, where a flexible bread folds a complete meal into one hand, and its relatives, the breakfast burrito and the migas taco, deserve their own articles rather than being crowded in here.