Each ingredient has an area for Nutrient Codes. These codes come directly from the USDA database. (If you don't see a code, since the USDA does not list every ingredient, then you can create your own Custom Nutrition).
This box lives on the lower left of every ingredient screen:
When you use that ingredient in a recipe we take the code and based on the quantity and unit used in a recipe we calculate the nutritional value of that ingredient and all other ingredients in that particular recipe.
This is a Prep Recipe for roasted salmon. Each ingredient has a USDA or Custom Nutrient Code:
We then use this Prep Recipe for salmon in a Serving Recipe:
At the bottom of that Serving recipe is a Nutrition Section. Using the USDA and Custom Nutrition Codes and based on the amount of each ingredient used we calculate the Nutrition Facts label: