Where to go through customs? Italy or France on the way home to the US

You will go through passport control at CDG; this is where you exit the Schengen area.

Thanks, any idea how much time we should allow to go through that process? It seems that 1hr35 will not be long enough.
Thanks.

I've flown FLR-CDG-SFO many times with Air France, often with very short layover. Sometimes I was the last one on board on the CDG-SFO leg before they shut the door behind me.
I'm assuming you will be flying AirFrance all the way.
The flight from FLR will likely drop you on the tarmac, where a bus will take you from the plane to the terminal. Sometimes the layover was so short, that AirFrance will meet us on the plane upon arrival, ask who was going to San Francisco, then, instead of taking the bus to the terminal with the other deplaning passengers, they would take us to a small van on the tarmac, which will zip through the planes all the way directly to the airside of the international terminal where the flights departing to SFO (and the US in general) was located (generally 2C or 2E). Once inside the terminal we would need to still go through passport control, before proceeding to the departure gate to the US bound flight.
What I'm trying to say is that AirFrance will go to great length to make sure you make that connection, because otherwise AirFrance will put you in a hotel at their expense (which they'd rather not do, because that is an extra cost to them). I always made it (sometimes, as I mentioned, barely) except for once, but that was because the flight to SFO was canceled and so had to depart the next day. I missed a day at work, but I got to spend an unexpected night in Paris at AirFrance's expense. I've been stuck in worse cities. Go for it.

You will go through Customs in the US. One hour and 35 minutes is plenty of time to get through passport control if you are flying on one itinerary and have checked your luggage to the final destination. As said above, it is in the airline's interest to make that connection happen. Now your luggage is whole different situation. I have made the second flight every time with a 90 minute connection but so far I am 1 for 3 with my checked luggage being transferred in time. It's not been a problem because the airline has delivered it to my house or hotel the next day.

I just get in line and don't overthink exiting the Schengen zone. Now that said, the French at CDG seem to move at a very methodically slow pace in getting travelers through passport control and security. A 90 minute connect is doable but not comfortable in my experience.

You really don't go through customs on departing the Schengen zone or a Schengen treaty country. However, you do go through passport control on departing the Schengen zone. You will go through immigration then customs when you enter the US. The US does not pre-clear from Italy or France.

The plane from FLR will likely arrive at terminal 2G, that is a small terminal quite distant from the other terminal 2 buildings. If you are not offered the airside connection, you must move quite quickly as you will need a bus shuttle to reach the departure terminal, and the move may need 20 to 30 minutes. The shuttle leaves in front of terminal 2G and arrives at terminals 2F - 2E; it runs every 5 minutes but the run is 15 minutes long.

I remember a time when I had a walking connection between two terminals in the 2 series (do not remember which ones), I had 1 hour and I made it with only few minutes to spare; with terminal 2G in 1 hour and half it still looks doable but you should better familiarize with the airport layout and know in advance where you should be going as you would not have time for missed attempts.

Interesting. My booked flight from Florence to Paris lands at 2F and my flight from Paris to Toronto is also at 2F.

Bobb Todd