Ciabatta con prosciutto e mozzarella is defined by a tension the bread has to manage: one of its two fillings is dry and salty, the other is wet and bland, and the ciabatta sits in the middle deciding whether the sandwich holds. Prosciutto crudo brings concentrated salt and savour in paper folds; fresh mozzarella brings cool, milky softness and a great deal of water. The combination works because each covers the other's weakness, the cheese rounding off the salt, the ham giving the cheese a reason to be there, but only if the ciabatta is right. Its open, irregular crumb and crisp shell are doing structural work, soaking a controlled amount of the cheese's moisture without collapsing, which a tight white roll could not do.
The craft is moisture management and the order of the layers. Mozzarella is drained and ideally torn rather than sliced wet, then sometimes laid against the bottom crust so its water is held by the crust rather than wicking up into the soft crumb. The prosciutto goes in loose, airy folds on top of or around the cheese so it does not compress into a salt slab, and because the ham seasons the whole sandwich, salt is rarely added at all. A thread of olive oil on the crumb is the usual extent of the dressing; the ciabatta's own char and chew supply the rest. Assembly is meant to be close to eating, because a mozzarella sandwich built an hour ahead is a soggy one, the crumb saturated and the crust gone limp. Eaten fresh, the contrast is the whole pleasure: crisp shell, yielding crumb, salt, and cool milk in one bite.
The variations are about what is allowed to join the pair without burying it: a few basil leaves and tomato, which tips it toward a caprese reading, or a smear of pesto to waterproof the crumb and add a third green note. Swapping in mozzarella di bufala changes the water content and the assembly entirely, and swapping the prosciutto for a smokier or spicier salume makes a different sandwich. Those, and the wider catalogue of fresh-cheese panini, deserve their own articles rather than being crowded in here.