I primarily use soybean meal for a protein source in my feeds. Soybean meal is 44% protein (which is very high for a plant protein). I usually use corn as my other primary ingredient. Corn has protein of 8%.
If I mix 1500 pounds of corn and 500 pounds of SBM, I end up with a mix that is 16% protein overall. I can adjust the ingredients to achieve different protein levels in the finished product. I adjust it for different types of livestock, as their protein needs are different. I also usually add other things to the feed, such as minerals/calcium/salt or whatever else is needed.
Protein sources for commercial feed are whatever is cheapest and legal for that type of livestock. It can be a variety of plant sources, meat and bone meal, fish meal, or the mysterious "animal proteins" which can be just about anything, including ground up chicken feathers or other strange things. They sell the feed based on the level of protein. It is usual practice to vary the ingredients depending on prices of various products, so what you get this time, might change by next time. The protein level will be the same, but what they use to achieve that is not.
If you know the nutrient requirements of the animal you want to feed, you can come up with your ingredients and simply request that they be mixed. If you buy pre-mixed feed, check your labels to see what's in it.
There are not too many fillers in feed. Every ingredient serves a purpose, whether it is to boost protein or just overall energy availability. Almost nothing is totally digestible, so the filler is in the undigestible portions of the feedstuffs. Soybean meal is 90% digestible (by cattle anyways), so there is 10% of just fluff.
Jena