My return from from Venice was booked from Venice to San Francisco with a stop in Rome. I am flying the same airlines. Will I need to go through customs in either Venice or Rome? Thank you.
Customs will happen in San Francisco. This is where you identify any items that you need to “declare”: agricultural products, items for resell, etc.
Immigration is different. This is where they check your passport. This will also happen in SF, but could happen in Rome also. You will likely have to check in at the airline counter in Venice and have your passport checked there.
You leave the Schengen zone from Rome. So exit immigration at Rome. At San Francisco you will go though US immigration and customs.