We tend to forget how much things have changed.
This movie shows us how book printing worked until recently. The technology is letterpress. This was still the process until near the end of the 20th century, even into the 21st century a bit. Thanks to Gary McLaren for finding it and sharing it.
It’s still what the workflow looks like in tradepub, and one of the reasons they want to take such a huge hunk of the royalties. On-demand self-publishing uses printing-grade copiers which automatically arrange the pages on the sheets [four to eight pages on each side of the 8-page or 16-page signatures]. But the binding, trimming, and covering are still very similar—for hard covers. The perfect-bound [square edge] books are a little easier to merge with their covers for a one-off work flow.