Hi Griffins,
It is always the first place you land at, so it will be JFK.
You will also need to collect your luggage to take it through customs and then you need to take it to the connecting flight area. Some airports you have them as soon as you leave the main exit door, or you have to take it to main check in for said airline.
Once at JFK you will have your passport and documents checked and you will be asked at customs if you have anything to declare etc
Some airports have even had extra check points for extra security before you get out of the exit door. (eg: another xray machine or further questioning on what you have in your luggage.
Going back out of the country, once you check in your luggage you won't see that until your destination or 1st foreign country you land at. So when you connect flights, there is no need for collecting luggage again.
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