The panino con michetta is defined by an absence: the michetta is a Milanese roll that is mostly hollow inside, a crisp star-shaped shell of crust around a pocket of air. Press it and it gives, then springs back; tear it and the interior is more cavity than crumb. That emptiness is engineered, not a flaw. It means the roll weighs almost nothing, the crust does all the structural work, and the air pocket is there precisely so a filling can be tucked into it without the bread fighting back. As a sandwich the michetta is split through its equator, the pocket opened, and a modest filling slid inside, and the eater experiences crust first because there is so little crumb to get through.
The craft is timing and restraint. A michetta is at its right state for only a few hours: fresh, the shell shatters and the cavity holds its shape, but it goes stale fast precisely because there is no dense crumb to keep moisture in, so the honest version is filled and eaten the same morning. The hollow interior sets the rule for what may go in it. A wet filling has nothing to be absorbed by and will simply pool and then soak through the thin shell, so the classic loads are dry and few: a little salame, prosciutto, a slice of cheese, perhaps a thread of oil. The roll is not buttered, because the appeal is the dry crack of the crust, and the portion is kept small so the structure stays intact in the hand.
The variations are about the filling slid into the pocket rather than any change to the roll, which is fixed. There is the salame reading, the prosciutto reading, the cheese reading, and the bare michetta split and eaten plain with the bar. Each is the same hollow shell met by a single dry filling, and those deserve their own articles rather than being crowded in here.