Complex sentence: It was through Schwartz that Kubrick acquired the stock footage of the explosion which ends the movie, Schwartz arranging for it to be delivered to London through a contact Milton Caniff had in the USAF.
Split into simpler sentences: It was through Schwartz that Kubrick acquired the stock footage of the explosion which ends the movie. Schwartz sourced and arranged for it to be delivered to London through a contact Milton Caniff had in the USAF.