Contrary to most other countries, there is no sterile international transit in the US.
All passengers arriving in the US, whether it's their final destination or they transfer to a domestic or international flight, will have to:
- go through passport control (and thus have a valid US passport, green card, visa, or ESTA),
- reclaim luggage
- go through customs.
After customs, if you are on a single ticket and checked through to your final destination, you will be able to drop off your bags (already tagged for your final destination) at a counter just after exiting customs. You can then go to departures, through security, and to your gate (possibly changing terminals in the meantime).
If you are on separate tickets, you will have to go the the check-in area for your flight to check-in and drop your luggage before you can go through security and to your gate.
There are two exceptions:
- If you arrive from a "pre-clearance airport", where checks will have been done at the departure airport, you will be considered as arriving on a domestic flight. But this does not apply for Frankfurt at this time.
- If you transit in MIA or DFW and you arrive from selected airports on selected airlines and you got red luggage tags, you will still go through passport control and customs, but you will not need to reclaim/recheck luggage. There's a pilot in ATL for flights from ICN (!) to do the same thing. But none of those apply to you.
Note that in all cases, since you will be entering the US, you need to meet all requirements for entry into the US, not only from an immigration point of view (as described above, US passport, green card, visa or ESTA), but also for Covid-related measures (vaccination and recent test).