From my estimate, they produce plastic resin in pellet form. Imagine a material that is like coarse plastic sand.

If you are making a respirator mask, another factory will take these pellets, heat it to melting, and basically spray the hot plastic to form a sheet with very fine plastic fibers. Because the fibers need very specific properties, they probably use a custom plastic resin, not a regular commodity resin.

If you are making protective clothing, a different factory will take the pellets, heat it to melting, and slowly extrude it through a nozzle. Once you get a sheet, you cut it to size, and maybe glue some different pieces together. https://www.youtube.com/watch?v=W6V0w-ajtN8

From watching a video about this yesterday, I think they manufacture polypropylene pellet stock suitable for both injection molding and the various melt-blown processes that make nonwoven cloth. This pellet stock is in short supply worldwide and much of it is made in China.

They're the first link in the supply chain for many plastic and polyester products.