The birria marinade is a key component of the dish's delicious taste. It's known as an adobo that typically includes ingredients like dried chilies (such as guajillo that give the red color, ancho, and pasilla), garlic, onions, vinegar, and various spices like cumin, Mexican oregano, and cloves. You'll need to rehydrate the dried chilies, blend them with the other ingredients and some soaking liquid, and then cook it low and slow until fall apart goodness happens.