You will go through passport control in Paris before your flight to Milan. You will change terminals and then be funneled into the passport area. You will have customs in Milan but you will likely not even notice it as you will just go thru a door that is green and where everyone else is exiting. If you are flying Delta you will land at 2E and then walk to 2F for your Milan flight.
On the way back home you will go thru Passport control in Paris to exit the Schengen area. When you get to MSP you will go thru both Passport Control and Customs there.
How much time do you have in Paris going and coming back? It took me 1h10 minutes from when the door opened on my Delta aircraft in September to get to the gate for my Milan flight. The flight to Milan was completely full and they started boarding before the time listed on the monitor.
Have you already bought the Milan to Venice ticket? If not, it may be quicker on the train.