It varies by route/connection/destination.

On many int'l-to-int'l connections, you don't clear immigration or customs at the transit point...but you *do* have a doc check and a security checkpoint. Thus everyone milling about in the international airside area has been "secured" per local standards, whether they are local or connecting.

In some cases, like an international arrival into the Schengen zone connecting to a short hop within the zone, immigration and a security check happen at the point of entry, but customs occurs only at the destination.