Meat by-products are a bit of a consfusing topic when it comes to dog food.
The most expensive animal protein, is "muscle meat" — which is what us humans typically eat, e.g. chicken breast, beef steak cuts, etc. This is often the highest quality meat due the fact that most of it is destined for the human food chain. Organ meat, which is technically a by-product, is some of the most nutrient-dense parts of the animal. That being said, since a lot of organ meats and other parts of the animal (such as leftover bones and cartilage) are not destined for human consumption, they are packed together and treated with chemical and turned into powders and gel-like liquids that can then be used in pet food processing. We only used named organ meats and muscle meats in our diets. Additionally, all of our meat comes from animals intended for human consumption and none of the meat is chemically treated - it is fresh or frozen right until it is added to our recipes.