Canon manufactures no lenses in the USA. The "US" versions of each Canon lens model are made in the same plant that all of the rest of that same lens model is produced and are physically identical within a particular production run. To the best of my knowledge, all Canon EOS lenses have the country where they are assembled printed on either the front or rear of the lens. But EOS lenses made in a particular country can be assembled from components that have been produced in various other countries. The older lenses made before the EOS system was introduced in 1987 were, as far as I am aware, all made in Japan.

The reason there are "US" and "International" versions of Canon lenses has to do with marketing, controlling the supply of lenses to a particular country, and currency exchange rates. "Gray market" or "International" lenses purchased in the US are physically identical to "US" lenses that were imported via official Canon channels, but the gray market lenses were purchased from authorized sellers in other countries and then imported into the U.S. via unofficial channels as far as Canon is concerned. Where there are differences in warranties between "US" and "International" versions of the same lens models, it also has to do with marketing decisions as well as compliance with consumer protection laws in the various jurisdictions where Canon products are sold.