In GVA if you're arriving from the US and want to exit on the French side of the airport, the process is that you go through Swiss immigration, collect your bags, walk through customs and you'll be in arrivals on the Swiss side. Go up one level to check-in, walk to the north-east end of the terminals (if you're facing the check-in desks, turn right). At the end of the terminal turn left and walk through some one-way gates, past French customs (who are rarely present) and you're in the French sector. There are no immigration checks between Switzerland and France as both are in the Schengen zone.
There's no direct access to the French sector from planes arriving from destinations outside France. Note also that the French sector is very small but has the advantage that rental cars are directly accessible, without the need for a shuttle.