It was not only trim, but also the garments themselves that were made in mass. Mass production started out to mean people would take in a section of, say a blousewaist, in their homes.
They would make hundreds of collars in their cottage, and then carry them to the factory in town. The factory would just do assembly. This created a whole industry of transport and carriage as well as garments.
As time went on, and late in the 1880’s in the U.S., the people went to the city and worked inside the factory.
(Photos: undergarments were still handmade in the 1830’s, yet by 1910, they were almost all mass produced and sold in catalogs as these two wedding ensembles illustrate).