A few weeks ago, I was on a LAX-LHR flight that got diverted to JFK. I assumed, as we were being diverted, that we would have to clear immigration upon landing in New York. I thought that because there were many people on the aircraft who had returned their I-94 in LA.

Yet, we were dropped at JFK directly into the domestic terminal. So these people had left the country without really leaving the country.

Then I remember reading somewhere that a US aircraft is US soil. So does that mean that I am on US soil until I step off the plane say in London?

That doesn't sound right either.

When do I actually exit the US?