Budgie parakeets are in the wild a vegetarian-leaning omnivore. They will opportunistically consume insects or other small animals that get on the foods they eat (grasses, tender buds and shoots, flowers, grains, and tree seeds) but they do not actively seek out animal sources for food. Budgies in captivity are often fed "Seed mixes" which consist of millet, canary grass, oats, oilseeds (e.g. rapeseed, safflower, sunflower hearts, soybeans and often dried root and leaf vegetables. This is not necessarily a complete diet for budgies, and this page will go over various aspects of diet.
Seed mixes for budgies ignore the science of budgies who eat in the wild. In the wild, budgies eat much more green seeds (e.g. fresh, undried seeds) as well as plant matter, incidental insects, and flowers and such. Dried seeds are nutritionally different, starches in particular are higher. Per 100g portion, millet is 73g carbohydrates, 4.2g fat, and 11g protein. It is not, as people would give you evidence to believe, high in fat. (Citations at the bottom of page). Canary grass is ~20% protein, ~55% carbohydrates, and ~7% crude fat. These two make up 80-95% by weight of most seed mixes. Oats and wheat are also low in fat. Roudybush Pellets are labeled around 11% protein, and 6-8% crude fat depending on variety (No citation, see bag labels). Harrison's Adult Lifetime Super Fine labels "Crude Protein (min.) 14%, Crude Fat (min.) 6%, Crude Fiber (max.) 5%, Moisture (max.) 10%, Omega-6 Fatty Acids (min.) 2.8%, Omega-3 Fatty Acids (min.) 0.35%." on its puchase page. Under US Law, labeling for bird products is not required to follow human nutrition facts labels. Unfortunate, as this would help clear up much problematic usage.
Seeds are less of an issue in their macro nutrients, and moreso in the lack of essential vitamins and minerals. Calcium, B-vitamins and vitamin E are all essential and yet are often deficient for budgie parakeets on seed diets, in my experience. This can lead to strokes, eggbinding, and other life threatening conditions.
There is no one-size fits all answer. Most bird formulated foods (Commonly called pellets, kibble or "bird food") in the US are well-formulated for the basic needs of budgies, at least those marketed for budgies. However, it's known that budgies are not easily trained to eat these products, and it can be difficult to convert them. The definitive, though, is: