I know this has been asked, and I've read several old threads here and at other sites, but I am still not sure if I understand things correctly.
Direct flights are not available on my travel dates, so I am looking at a few flight options with different airlines. For the outbound itineraries from the US, a couple of plans have us transfer to a second flight in Toronto, Dublin, or London. The return options are the same: stopping in London, Dublin, or Toronto to connect to the second flight back to the US.
To reach my second flight in either direction, will I have to clear local customs at Toronto, Dublin, or London? I understand that upon return, I will clear US customs if my connection is at a US airport, before getting onto my final leg.
I'd like to understand this to make sure that I account for enough time on the ground during my intermediate stops. I've done this via Heathrow/Gatwick before, but honestly can't remember anything about going through customs. I've also gone through Shannon, and definitely didn't go through customs, but that was a military charter so who knows.
Thanks in advance. I know this is probably a dumb and/or played-out question, but I've read and heard conflicting things.