The Frito pie sandwich is defined by the one ingredient most sandwiches work to keep out: a layer engineered to stay crunchy in a flood of wet chili. Frito pie is corn chips under chili, cheese, and raw onion, and moving it onto bread or a bun does not change what it is. It adds a second carbohydrate whose only job is to give the hands a grip while the chips inside stay crisp as long as they can. The bread is the wrapper. The corn chip is the structure.
The craft is a race against sogginess. The chili has to be thick and tight, a fine-ground Texas-style red with little loose liquid, because a thin chili turns the Fritos to mush before the sandwich reaches the mouth. The chips go in dry and stay layered rather than stirred, so the chili sits on top and the crunch survives a few bites instead of none. Shredded cheese is added hot enough to slump and bind the chips into the chili so the structure does not simply slide out of the bread. Raw chopped onion is the sharp, cold counter to a rich, salty, fat-heavy filling that would otherwise read as one note. The bread itself is chosen to disappear: a soft white slice folded over, or a split bun that soaks the chili it cannot stop. This is concession-stand and lunch-counter food, assembled fast and eaten immediately, because it has no shelf life at all once the chili meets the chips.
The variations are mostly about the carrier and the chili. The walking version skips bread entirely and serves it in a split chip bag. A bean chili, a brisket chili, or a green chile build swaps the filling while the dry-chip rule holds. These belong to the dense regional shelf of place-tied American sandwiches, and each deserves its own article rather than being crowded in here.