Dog food is a complicated subject. Many of the larger companies are not using the most healthful ingredients. Try reading the ingredient list on most pet food labels and you will see a lot of stuff you cannot identify. Here is a link to The Dog Food Advisor. It is a start in understand some of the ingredients. You can take or leave the ratings but at least you will have a better understanding of what is in the food.
Dry Dog Food Reviews | Dog Food Advisor
After finding out that all kibble is cooked at extremely high temps at least
3 times in order to kill all organic matter in the food I decided that it didn't much matter. You will see a long list of synthetic vitamins at the end of all the ingredient lists. These are necessary since the original nutrition in most of the foods has been cooked out of it. For foods to be able to sit on shelves for months at a time they cannot contain organic materials.
There are other sites to learn more about pet foods if you are interested. I agree it is difficult to make a choice.