>The question I have of Curtis is whether he starts with a premise and then finds video to support it, or whether he derives his ideas from watching random archive video, or both. It would be fascinating to watch a "making of" documentary that shows how he finds the right archival footage.

You should read the article then, as it is discussed in there. He has access to the complete BBC archives and just scrolls through material on fast forwards until he stumbles onto interesting threads that he saves and then later finds patterns in.