I have some beer glasses with decal-like decorations that I assume are some kind of plastic or maybe even glass/faience. How are these made?
This is most likely an applied ceramic label. It's an industrial process that can't be achieved in a typical handmade craft setting (e.g., your garage), requiring ceramic inks or enamels, screen printing to apply the design, and high-temperature baking to set.
If you're considering removing an ACL to decorate the beer glass differently, the verdict is apparently that it isn't really removable (see related Q/A).