Often, they are made exclusively of peppermint leaves.
Sometimes they contain peppermint leaves and other herbs, such as other mints (like spearmint) or other non-mint herbs.
Sometimes they contain flavorings made from the extracts or distillates of peppermint or other plants…these flavorings are usually labeled “natural flavors”. “Artificial flavors” include synthetic flavorings that are not derived from a natural plant source.
If you are concerned about what is in your peppermint tea, read the ingredient list. In many climates, including most temperate climates, peppermint is very easy to grow, and is available for sale at most nurseries and garden stores.