Transit in USA
 

Does one need to clear immigration when transiting in US? For intl to intl flight at DFW.

Although I am US pp holder but I have never transited through US. Just wondering.... how does it normally work?

All passengers must clear immigration and customs (must reclaim all bags) - there is no transit.

Immediately after exiting the customs hall, there are baggage counters where your airline will re-check your bags on to the final destination.

You are now landside and will clear the standard TSA checkpoint to continue your journey.

Gotcha.

Just out of curiosity, what to do non US pp holders do? Get a visitor visa and clear immigration?

It depends on their nationality. Citizens of many countries do not need visas, only a passport.

They still need an ESTA. Which is like a cheaper online Visa, which can also be arbitrarily denied. Only exception I know of is Canada.

Or better still, avoid USA "transits". People just select flights that do not land in USA. Go via Canada, South America or the long way to their destination.

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TWA884
Co-moderator, Travel Safety/Security.

Besides Canada, the other exceptions are Bermuda, the Bahamas, and some other random islands in the Pacific.

To answer OP's question, everyone needs to go through immigration and customs upon landing in the US. There is no "transit zone" to directly board another international flight.

Canada isn't any better, since you need to clear Canadian immigration and customs. Nicer people than their US counterparts, but still a hassle.

International transit in Canada does not require passing through immigration (unless the destination is USA in which case US preclearance is processed in Canada).

Oh yes it does. In YVR it's fortunately a completely separate line, which was empty when I was there. But I still had to have a Canadian ETA, fill out the Canadian landing card, and show my passport to a CBSA officer before continuing.

Similarly in Toronto I saw some closed CBSA International transit desks, with signs pointing to the next transit desk (which I didn't see since I was going to preclearance).

After that there's security depending on country of origin. Originating from the US I was allowed to skip security, I'm not sure what other countries are considered friendly.

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Please let's stay on topic:
Thank you,

TWA884
Co-moderator, Travel Safety/Security

Another question on-topic...

if someone is just trying to do an international transit and for some reason CBP do not provide entry in US, what happens? They gotta go back or they are still allowed to continue their towards destination?

Generally the airline will not let you board at the departure airport unless you have the required visa and/or documentation.
From the TV programs on border security, people get sent back if there are problems with admission into a country.

There is NO "International Transit" in the US.... (well.. NZ has/had a special arrangement at LAX for the flight continuing to LHR but thats the only one I know of...)

So that person is treated the same as anyone else. Onward plans would not be relevant.