Will I go through immigration at JFK or Niagara?
If I finish immigration at JFK, can I leave the airport to visit New York and then board the flight to Niagara? Do I need to do anything extra when boarding my flight to Niagara, such as rechecking my luggage?
Can I choose to stay at JFK airport and board my flight to Buffalo without going through immigration at JFK?
As others have explained, you will have to go through immigration (passport/visa control) in JFK, as that is your port of entry into the US and you are not serving for a pre-clearance airport. You definitely cannot do that in BUF.
Now, considering all your flights to be in the same ticket, the usual procedure for a layover would be:
- At the origin, your bags would be tagged to your final destination (BUF)
- You would receive boarding passes for 3 flights
- On arrival in JFK:
- Get off the plane
- Go through immigration
- Retrieve your bags
- Go through customs
- Right after customs, drop your bags at the counter for connections (bags are already tagged, so you really just drop them on the belt) — at this point you are in the public area of the terminal, you can go anywhere you want
- Go to your departure terminal
- Go through security
- Go to your gate
- Board your connecting flight
Now, given the quite long overnight layover, it’s quite possible you won’t get all boarding passes at the origin (or in SIN), which means you will need to get it in JFK (though I expect you should be able to get a mobile boarding pass on your phone).
Most importantly, you will want, especially after the extremely long SIN-JFK flight, to spend the night in a real bed in a real room before your next flight.
For this, you have two options:
- you can ask them to short-check your bags to JFK only, so you can retrieve them and bring them with you to a hotel (you will retrieve them anyway, but I’m not sure what happens if they expect them to be dropped at the connection counter and you don’t, so it’s safer to ask). You will need to go to a check-in desk the next morning to drop your bags (so don’t forget to be there early enough)
- You can drop your bags for the connection and just have the required things for the night in your carry-on. Remember that there are limits on sizes and quantity for liquids, so plan accordingly for toiletries.
In other circumstances I would advise you to spend the evening in Manhattan or other interesting places before crashing for the night, but you are probably going to be extremely tired at that point.
A hotel close to the airport is probably the best option. It will usually be cheaper than in central locations, and you will spend less time travelling to/from the hotel.
With a few exceptions all passengers arriving in the USA go through immigration at the first airport they land at. Most of the exceptions are for people who already go through immigration before they get on the flight that gets them into the USA. The last exceptions are for very few people who arrive and leave the country from very few selected airports and flights.
You will never been given the option to skip immigration when everybody else does it to go on a next domestic flight, as they will not be able to track you and make sure you do not skip immigration that way, this is not just the USA but all countries. It is also very likely that there is no immigration at the airport you go to next, so that would be a no go anyway.
After you go through immigration you have to pick up your luggage, walk it through customs (although in the USA both immigration and customs are done by the same service) and drop it off for your next flight, even when it is tagged for your final airport when you hand it in, again this is the way it works in the USA. When you have done customs (and the drop off of your luggage) you are free to leave the airport. Whether you can drop off your luggage when you arrive depends on whether you booked the flights as one and whether the airline accepts your luggage at that time. (I think they should if you booked it as one flight but I have never done a connection in the USA myself.)
If you can not drop off your luggage you might find a luggage storage point but I have heard US airports do not always have them and going sight seeing with big luggage is no fun, so in that case you may want to stay in the airport, otherwise you can go wherever you want as you have been allowed to enter the USA, (which would also be the case if your next flight was out of the country.)
With an arrival in the early evening and a departure in the morning I would look for a hotel, likely one with easy access to the airport in the morning, (and if you do, you have a place for your luggage if you can not drop it off.)
You will go through customs and border protecrion / immigration (CBP) where you arrive in the U.S. first, i.e. JFK international arrivals. Some other airlines, but, I believe, not Singapore, let you go through customs and immigration before departing from some foreign airports, so you come to domestic arrivals in JFK, which is more convenient. You can ask Singapore Airlines to confirm. E.g. https://www.etihad.com/en-us/fly-etihad/us-immigration-in-abu-dhabi works this way.
Yes, once you pass CBP, you can leave the JFK airport, which is already in NYC, but you probably meant visiting the more touristy parts of NYC in Manhattan. Mass transit (e.g., Air Train to Jamaica Station, then Long Island Rail Road to either Grand Central Madison or NY Penn Station) is a faster and more reliable way to get there and back than a taxi.
It is possible, but unlikely, that Singapore will pass your checked in luggage on to Jet Blue. That would be quite unusual, but you can ask Singapore Airlines to confirm. A much more likely standard operating procedure is than you retrieve your luggage from a carousel in Terminal 4 soon after you arrive, lug it yourself to Terminal 5 (you can walk directly over; or you take the Air Train, which might actually involve a longer walk), and give it to Jet Blue shortly before your connecting flight.
But you don't want to take your bags with you when you visit Manhattan. Contrary to what another answer says, there are some services in JFK that let you store your luggage: http://www.jfk-airport.net/luggage-storage.html However not all of them are open 24 hours, so make sure that the one you use will be open when you want to get your luggage. So, you probably want to store your bags in T4, visit Manhattan, return to T4, retrieve your bags, carry them to T5, and give them to Jet Blue.
- Extremely unlikely, given that you're going to a different JFK terminal, and taking a domestic flight where no one will go through immugration. But you can ask Singapore Airlines.