The bean and cheese taco is a San Antonio breakfast object, and what defines it is the flour tortilla, not the filling. This is a wheat tortilla, soft and pliable and slightly chewy, not a corn one, and that choice sets everything else. A warmed flour tortilla folds without cracking around a hot, loose filling and holds it for the length of a one-handed breakfast eaten standing up or in a truck. Refried beans and cheese are about as plain as a filling gets; the engineering that makes it work is the bread wrapping a wet load and not failing.
The craft is in the refry and the heat. The beans are pinto, cooked down and then fried in fat until they go from a side dish to a thick, spreadable paste, which is the whole reason they work here: a paste binds, a soupy bean does not, and a binding filling is what keeps the taco from blowing out its seam. The tortilla is heated on a comal or flat-top until it is supple and just starting to blister, then the hot beans go down and the cheese, usually a mild yellow shred, melts against them from the residual heat rather than under a separate broil. Order matters: cheese against the warm tortilla and beans, so it tacks the fold shut as it softens. The fold itself is the structural decision, a single turn rather than a sealed cylinder, which is why this is a taco and not a burrito and why it relies on the eater's hand and a tortilla with real give. At a San Antonio taqueria these come off the line in seconds, wrapped in paper or foil, built for volume and for being eaten before they cool.
The variations are small and additive, which is the point of a baseline this plain. Bacon turns it into the bean, bacon, and cheese; a spoon of salsa or a few slices of pickled jalapeno adds the acid and heat the base deliberately leaves out; chorizo pushes it richer. Each of those is its own taco with its own balance and belongs in the broader taco and breakfast taco family rather than crowded in here.