The panino con coppia ferrarese is named for its bread, and the bread is unlike any other carrier in the Italian repertoire. The coppia ferrarese, the protected sourdough of Ferrara, is shaped into a twisted four-armed form with a dense central knot and four thin curled horns that bake bone-dry and crackling-crisp. It is mostly crust. The crumb survives only in the knot at the centre; the arms are almost entirely shell. Building a sandwich on it is a deliberate choice to lead with crunch, and the filling has to be sited where there is enough crumb to hold it rather than spread across a bread that is largely air and crackle.
The craft is dictated entirely by that geometry. The coppia is split through its dense middle, the only part with crumb enough to take a filling, so the sandwich is small and concentrated rather than long and stacked, and the brittle horns stay as the crisp handles on either side. The filling is kept dry and restrained, a few slices of a regional salume or a firm cheese, because any moisture would soften the very crispness that is the bread's entire reason for being and a wet build would turn the shell to leather. The contrast is the point: a hard, snapping crust against a small soft centre of cured meat. The bread is doing the textural work that a filling does elsewhere, which is why the filling is allowed to be plain.
The variations are a matter of what goes in the knot, each its own preparation rather than a footnote here. There is the salame ferrarese version local to the city and the one filled with the soft regional salama da sugo worked spreadable; both are the same twisted bread meeting a different Emilian larder. The wider family of bread-named regional panini, the ciriola and coppia and their kin, works on the same principle that the loaf is the signature and not the filling, and each deserves its own article rather than being crowded in here.