Whey is a by-product of cheese production. Whey protein is produced from whey after removal of sugars (lactose) and lipids. Protein fraction consists mainly of beta- lactoglobulin, alpha-lactalbunim, bovine serum albumin, immunoglobulins and several other protein groups. Whey supps differ in purity (concentrate, isolate, hydrolysate) and processing that takes place during the production. Because, as you can guess, whey protein actually has many active ingredients, both purification and processing can change how whey supplements affect the body (so talking about the effects of whey protein is quite generalized and not very specific. Regarding active ingredients, this is actually an interesting question because nutritionists (also doctors,...) tend to oversimplify food and think about it in terms of macro (protein, carbs, fats) and micro (eg. minerals) nutrients, while we know that various foods have several hundreds of active ingredients that all affect the organism that consumes the food. Whey protein is not an exception, there are several different active ingredients in whey that affect both our health and "growth". For example you can look up specific fragments of several proteins found in whey that have different biological effects eg. lactoferricin, albutensin, lactorphines,... It is quite possible that whey affects growth through some identified and some yet unidentified peptide fragments. Apart from that, you should definitely think about some more basic stuff like insulinogenic amino acids (such as leucine), and amino acids in general, as protein building blocks.

Ok, I think that you need to understand more about muscle. The most important ingredient is carbohydrates to develop muscle mass. I know that protein is necessary , but if you have enough glycogen your muscle develops better. I suggest you see the book "Clinical Sports Nutrition" by Louise Burke.

On the other hand, some important nutrients that are added to whey protein for better results are: glutamine, HMB, and Creatine. In my opinion, HMB is the best to improve muscle mass. HMB is a metabolite from Leucine (a BCAA) and it shows important results about muscle mass.