Customs questions

Dublin has a special arrangement; it is rare.

What airline are you flying and do you have any connections? As mentioned, Dublin has a special arrangement, so do cities in Canada. If you have a connection in Montreal, for example, you will do your passport control there before departing for the US. If you're flying non stop from Rome, you will do passport control and customs in your arrival airport in the US.

We are considering our flight options right now. We could connect in Atlanta if we fly from Rome, but the closest airport is actually Florence since we will be staying in Cortona. If we leave from Florence, there is a noon flight but it has two connections the first at CDG in France and the other in the US. The layovers at CDG are only a little over an hour-and-a-half and I worry if we must do customs there, we might miss our connection. I guess I am just trying to understand exactly where I should allow the extra time for customs.

US Preclearance facilities:
https://www.cbp.gov/travel/preclearance#:~:text=Today%2C%20CBP%20has%20more%20than,Ottawa%2C%20Vancouver%2C%20Victoria%2C%20and

Today, CBP has more than 600 officers and agriculture specialists stationed at 15 Preclearance locations in 6 countries: Dublin and Shannon in Ireland; Aruba; Bermuda; Abu Dhabi in the United Arab Emirates; Nassau in the Bahamas; and Calgary, Toronto, Edmonton, Halifax, Montreal, Ottawa, Vancouver, Victoria, and Winnipeg in Canada.

Customs is most likely going to be in the US. It should not really even be a factor in your decision if you are booking the whole thing through one airline (on one ticket), as they are going to book you layovers that you can realistically make. Anything over an hour and a half is usually fine for a layover. I try to avoid London but most are fine. May is not July and should not be slammed like last summer.
I would try to limit it to one layover, even if you have to get to Rome the night before a flight. Changing planes two times just introduces too much potential for chaos.

Customs and Immigration/Passport control are separate processes, the former being mostly passive these days.

Arriving in Schengen Zone Europe you'll go through immigration at your first airport of arrival, and customs as you leave your final airport (providing there was a connection)

A noted, Pre-Clearance into the US is rare: only from Ireland, Canada, and AbuDabi (maybe one more...?). Most often you'll go through both immigration and customs in the first US airport you land into.

"The layovers at CDG are only a little over an hour-and-a-half and I worry if we must do customs there, we might miss our connection. I guess I am just trying to understand exactly where I should allow the extra time for customs."

You will go thru Border Control/exit IMMIGRATION in Paris. This will clock you out of the Schengen zone where you as a US passport holder are restricted to 90 days. I am very risk averse when it comes to catching a flight back to the US. I'd want 2 hours here but I am sure many make it in 1.5 hours. Is there an earlier flight out of Florence?

Without trying to sound too pedantic:

Immigration - the process for people to move in and out of a country.

Customs - the process for goods to move into a country. You don't have customs when you leave a country. You do have customs when you enter a country. For Europe this is mostly walking thru a green-colored door or "lane". For the US, you often have to stop and talk to a Customs officer, depending on Arrival airport. They will ask you how much stuff you've brought back.

To make it simple.....if you fly to the US from either France or Italy, you will go through US immigration and customs at your first port of entry into the US. For you that would be Atlanta.

Thank you so much everyone. I really appreciate all the helpful information.

Here is a video on what to expect when clearing customs in Italy

https://youtu.be/IcaO92v9XK8

Hahaha, Roberto!!

Actually, as I understand it, the op is asking about customs upon return to the U.S. And, no, you wouldn’t do it in the city of departure. Customs is about bringing goods into the country. Our experience has been that on the plane we have been given a form to fill out, which asks about our purchases, mainly alcohol, cheese, meat, and a few other types of items. Upon arrival, an inspector actually checks the form and may look at questionable items. Once, for example, we had been given a can of foie gras pate. The inspector read the ingredient list and would have confiscated the pate had it contained pork.
Usually, we have structured our flights so that if a plane change is needed, we do it in Europe. However, occasionally a U.S. change has been needed. In that case all luggage is brought off the plane, and the described process occurs. Ongoing passengers then bring their bags to designated carts from which the bags are reloaded onto the plane or to the plane designated on the tag.

There is no longer a customs form when you enter the US, FYI.

Are you referring to Customs, where you declare goods, or Immigration, where passports are shown?