Cook tomatillos, tomatoes, serrano peppers, chile de arbol peppers, and 3 whole garlic cloves in a saucepan over medium-low heat until mixture is juicy and peppers are tender, about 10 minutes. Remove and discard garlic cloves.

Pour mixture into a blender and blend until smooth, about 1 minute. Mince 1 garlic clove and mix into sauce. Season with salt and set aside.

Mince remaining garlic clove and place into a skillet. Add pork, cover the skillet, and cook over medium heat, stirring occasionally, until pork is no longer pink and the juices run clear, stirring often, about 30 minutes. Pour blended tomatillo sauce into the skillet, reduce the heat, and simmer, stirring often, until pork is tender, 5 to 10 more minutes. An instant-read thermometer inserted into the pork should read at least 145 degrees F (63 degrees C).