Corrugated board is manufactured in a "continuous" or "factory-built" process that is similar to manufacturing millwork for buildings and home construction [but nowhere near as fancy]. Paper, sometimes with additives such as waxes, glues, and inks, is fed into an elongated machine called an "Archimedes" headbox (a few companies still use the original equipment manufactured by Archimedes, Inc).
A stream of water provides pressure as the paper is squeezed through a shaped opening that forms the tube. The tubes are then flattened and cut into appropriate widths to create linerboards or corrugated mediums, which are then cut to the proper lengths. The linerboards are run through a machine called an "airplay" headbox. It resembled the Archimedes machine but used air to blow the fibers apart as they were formed into tubular shapes.