Miso paste is made from soybeans. The soup almost never is just miso. Miso soup is made on a dashi base, usually bonito or sardine, in shops and homes alike. Supermarket miso is often sold with dashi already mixed in: the pack says だし入り. Set meals, ryokan breakfasts and instant cups include it by default. Kelp (昆布) or shiitake (椎茸) dashi is plant-based. If that's the answer, enjoy it.