Hello,

I understand that when flying from Canada to the USA, the border checks are being done before departure in Canada due to the preclearance agreement.

What happens if you come from a third country and are denied entry to the US when passing the US immigration on the Canadian soil?

Are you arrested and deported to your home country? If yes, by which authorities - US or Canadian ones?

Or are you simply allowed back into Canada?

Background: I have a friend who has a higher-than-usual risk of being denied entry to the US, and who is scared to travel there because of that. There are many stories of people being held by US immigration for days or deported to strange places.

Would a travel via a country with a preclearance agreement with the US (Canada/Ireland) help him avoid the risk of being held by CBP, since he would only land on the US soil after having passed immigration?

Thanks