Use google search to find topics on bike forums. This website's built-in search function isn't very good. use this format: site:bikeforums.net schwinn factory

Then just swap out the search term. If you are wondering about the companies that made them and their respective industrial centers, lots of discussion on the different large manufaturers. I'd recommend "raleigh factory" for British, "schwinn factory" for US, and "etienne factory" for French. If you are wondering about the actual manufacturing techniques "electroforg", "fillet brazing", "lug brazing" would get you started.