If you checked a bag, you'll have to collect it from baggage claim from the international flight. You'll need to clear customs and immigration. Next, you'll recheck your luggage for the domestic flight.

Yes, in the vast majority of cases, if you are flying from an international destination into the United States and have a domestic connection, you must collect your checked baggage at your first point of entry into the U.S. This is because the U.S. requires all passengers to clear Customs and Border Protection (CBP) with their belongings before they can enter the domestic flight network. After you clear immigration, you will proceed to the baggage claim area to pick up your suitcases. Once you have your bags, you walk through the Customs area. Immediately after exiting Customs, there is almost always a "Baggage Re-check" counter where you can simply hand your bags back to the airline staff (since they should already be tagged to your final destination). The only major exception to this rule is if you are flying from an airport with "US Preclearance" (such as Dublin, Toronto, or Abu Dhabi). In those specific locations, you clear US Customs before you board the plane, meaning you will not see your bags again until you reach your final destination in the United States, as the flight arrives at a domestic terminal.