Soup soy sauce is a byproduct of making doenjang. Traditionally, Koreans made their own doenjang at home, so they always had soup soy sauce on hand. It’s very strong, fermented, salty, flavorful, and full of umami, so it’s perfect for flavoring and salting soups and side dishes (which is why it’s called “soup soy sauce,” or “soy sauce for soups”).
In order to get good quality soup soy sauce, it should be made from good quality fermented doenjang made from meju blocks. Well-made homemade soup soy sauce can be kept indefinitely in onggi, and some Korean families have soup soy sauce that’s more than 400 years old! On special occasions they’ll use one ladles’ worth.
I was raised on homemade soup soy sauce, and I’ve never found a commercially-made soup soy sauce that satisfies me. You can try some brands in a Korean grocery store, and a passable substitute is regular soy sauce and salt or fish sauce, but homemade soup soy sauce is 10x better, if you can get it.