Traditionally in Japan, miso soup is made by adding miso paste to dashi stock. Dashi is made from kelp called Konbu and Katsuobushi, dried fish flakes. There is powdered dashi you can add water to make a quick Dashi.
If buying miso paste and making dashi or powdered dashi is too much, there is so-called Miso Broth. This particular Miso Broth by Imagine uses no dashi broth, instead, adding vegetables like Onion and Ginger to the Miso Paste, thus, making it vegan. All you need to do is just add any fresh vegetables, meats, or fish to make miso-flavored soup. You can also add some broth.