The panino con guanciale is built on the cured pork cut that Romans reach for when others would reach for pancetta, and the difference is the entire sandwich. Guanciale is the pig's jowl and cheek, cured with salt and pepper and air-dried: it is far fattier than belly pancetta, the fat soft and almost translucent, the lean a deep dark seam running through it, and the flavour rounder and more intensely porky. This is the cure that builds carbonara and amatriciana, and in a panino it is the same logic stripped to its simplest form, one assertive Roman cure and the bread it belongs with. The defining fact is the fat: soft, abundant, and the point rather than something to trim.
The craft is the slice and what it sits on. Eaten cured rather than cooked, guanciale is cut thin enough that the soft fat yields in the bite instead of reading as a hard band, and laid in loose folds so the lean and the fat alternate and air moves through it. Because the cut is so rich, the bread carries more responsibility than usual: a rosetta or a piece of pane casereccio with real crust gives the fat something firm to push against, where a soft white roll would just go greasy. Nothing wet is added; the fat is the sauce, and an acidic condiment would only mute the cure it took weeks to build. At most a turn of black pepper, which is already part of how the jowl was cured.
The variations stay close to the Roman pantry, each its own preparation rather than a footnote here: the guanciale warmed and crisped so the fat renders before it goes into the roll, the broader habit of building a panino around a single cured meat, which is the Panino con Salume and its own piece. Each is one cure given its bread, and each deserves its own article rather than being crowded in here.