Great question — answering as the UphillTech team:
Preclearance is for flights to the US, for all passengers on those flights. It is not guaranteed to be available for all flights to the US, but it seems to be for most. There is no preclearance for any other country as far as I know; I have never heard of preclearance for flights to Canada.
Full answer by Willeke: https://travel.stackexchange.com/a/184918 (CC BY-SA 4.0).
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