The fried bologna sandwich is defined by a cut and a curl. The bologna is sliced thick, a quarter inch or more, and the slice is scored or notched at the edge before it hits a hot dry skillet, because an uncut thick round of bologna domes up into a bowl as it cooks and slides off the bread. The score lets it lie flat and the heat does the rest: the surface browns, the fat renders, the edges crisp and frizzle, and a soft pink lunch meat becomes something with a seared face and a snap. That transformation in the pan is the entire sandwich. Cold bologna on bread is a different and lesser thing.
The craft is in the thickness and the pan. Sliced thin, bologna fries to a brittle chip with nothing left inside; cut thick, it browns on the outside and stays juicy and yielding in the middle, which is the contrast the sandwich is built on. It goes on plain soft white bread, never toasted, chosen because the bread's job is to blot the rendered fat and stay out of the way, not to add chew. Yellow mustard and flat dill pickle chips are the working counter, sharp and cold against the warm fat, and they go straight on the bread so the structure stays simple. The whole thing is a lunch-counter and gas-station build that has to come together in the time it takes one slice to brown, which is most of why it stays plain. A slice of American cheese laid on the hot bologna off the heat is the common addition, melting into the seared surface the way it does on a flat-top burger.
The variations stay inside that fried-thick, soft-bread frame. A thicker single slab cooked like a steak pushes it toward a fried bologna steak sandwich; a chopped-and-griddled build runs it like a loose-meat filling; a fried-egg-and-cheese version turns it into a breakfast sandwich. Each of those is its own codified build and deserves its own article rather than being crowded in here.