One theory is that Swiss cheese is like a layer cake: multiple layers are sliced from a block of Swiss cheese without holes, modified with a melon baller, and then re-stacked, and heat welded back together. This is wrong.
Propionibacterium freudenreichii produces CO2 during the fermentation process, creating pockets of gas, leaving voids called “eyes.” (Cool people in the know will also call P. freudenreichii culture “Props.”) Because of pasteurization, Props culture is added back into the proto-cheese afterwards. Other Swiss-style cheeses have eyes and Gouda can, too.
Try as I may, I can’t really tell the difference between Emmental and Swiss cheese, but the minutiae of cheese taxonomy can get quite heated.