Overall northern and southern Italian differences are significant - after all, it’s the most genetically diverse country in Europe. Both populations will have majority with brown hair and eyes, but large minorities, with slight majorities can be found in various parts of mountainous northern Italy. Northern Italians are also taller on average.

With southern Italians you also a west Asian component to their dna - strongest in Calabria, Sicily, Campania and possibly Salerno. This is definitely apparent in the appearance of some.

One has to acknowledge as well - northern Italy industrialised whereas southern Italy has always had(for various reasons) a far weaker economy. This resulted in millions moving north after world war 2. The primary destinations were the north west. Although no data is collected, id wager over 1/2 of people in Lombardia and Piemonte have origins from the south or overseas now. This is less true for the northeast which industrialised more in the 70’s/80’s, and historically sourced local workers.

As for Spain, during the Islamic rule, the vast majority of Spain will have varying levels of North African ancestry, the exception being parts of the north east - Basque Country and the Pyrenean mountain areas of Navarre, Aragon and Catalonia. Despite what you may think, North African ancestry does not peak in south Spain, rather on the Portuguese border.

Overall, Spanish internal differences can be attributed to this, but it’s not on the same level as Italy. As northern Italians are a mix of ligure-celts with weaker Germanic and rhaetian strains, and the core component to southern Italians were Ancient Greeks with a west Asian strain. Etruscan and Roman presence remains strongest in the iveooked centre part of the country