San Francisco is a little different. Many people think that Italian immigration was mainly to east coast cities, establishing "Little Italies" in NYC, Boston, Philadelphia, Providence, etc.

Originally, Italian immigration to CA exceeded that to all east coast cities, combined. It's hard to pinpoint why mass immigration occurs, but it's usually due to some local upheaval.

What seems most likely is that the massive immigration of the 1850's was stimulated by the Revolutionary War (Il Risorgimento) that founded the country of Italy in 1861. The struggle to establish the country began around 1814, so it was a long struggle. During that 1850's a lot of people emigrated to the USA from Liguria and Lombardy, although it It couldn't be said that they emigrated from Italy, because Italy didn't exist yet.

They didn't come from Venice, because even after Italy became a country in 1861, Venice was a part of Austria at that time. Venice didn't become a part of Italy until Italy agreed to support Austria during the Austro-Prussian War, and Austria gave Venice to Italy as part of the deal. Even when Italy was established by the victory of General Garibaldi, and they chose a King (Vittorio Emmanuele), they still hadn't conquered Rome yet, and and had to make Torino the Capital.

After a couple of months they decided Rome should be the Capital, and Garibaldi went ahead and conquered it and made it so.

Those early 1850's era immigrants to California had a huge effect on the USA of today. For example, one of them started the Bank of America, still one of the largest banks today. Someone named Gallo wrote back to his hometown in Italy that the rolling hills, the sunlight, the moisture, the soil, the mild climate was just like home, perfect for making wine. As a result, a massive emigration of Italians from that region occurred, and they established wine making in the Napa Valley.

The same happened with olive oil. People wrote back home that northern CA is perfect for olive oil production. To this day, California olive oil is usually better than olive oil made in Italy. The number of contributions to American culture in all spheres of life from the original Italian emigration to California cannot be overstated.

After the founding of Italy, for some reason, the government became very vindictive against the South, Naples in particular. They levied usurious taxes that drove the South including Sicily, into extreme poverty to the point of starvation. That started the second wave of emigration.

By 1890 the number of Italians living in east coast Little Italies equaled the number of Italians in California. The southern immigration ended with World War 1, when transatlantic passenger travel pretty much stopped.

Italian, emigrated afterward, but not to the USA in large numbers, due to quotas. That is why most Italian-Americans are 4-6 generations removed from Italy.

Italians still continue to emigrate. The city with the most Italian citizens is Rome. Within the last decade 600,000 young Italians have moved to England, which people joke about by saying that London is one of Italy's largest cities.

In San Francisco many Italians in the tech industry started arriving in the last 10 years from everywhere in Italy, not from any particular region. There is a very robust community of native Italians, not just a historic, "Little Italy."

An interesting side effect is what has happened to spouses. Almost all who've come are men, bringing with them wives with incredible education and experience. Many have a PhD in architecture, engineering, history, whatever. I know of one who was a Professor at the Sorbonne in Paris, a PhD in linguistics, and on the side she modeled for Vogue, and other top magazines. I know a few PhD wives who were engineers who helped design major buildings, who are now just giving Italian lessons, while their husbands work in Silicon Valley. Most of are working at routine jobs in SF that they are way over-qualified for.

So, the answer to your question is, when you meet an Italian in SF, you can't expect a tendency for them to come from a specific region. There are the ones of long ago Italian descent, but also a continuing new wave of native Italians.