Please explain to me why she is an aggressive dog.
There could be a lot of reasons for her aggression. 1) Genetics, since it can appear in the breed regardless of socialization which especially expresses at full maturity 2) Lack of socialization and impulse control, not blaming you for this. It could have developed if she had been separated young from her family and siblings 3) Fear issues 4) Resource guarding (you or her dog friend, your GSD) 5) Unstable temperament 6) Health issues.
There are a TON of reasons for why dog aggression appears. In her case, it could be one or all of them. My best guess, it is genetics and resource guarding with poor impulse control. The important thing now though is to manage and maybe counter condition her reactivity/display into a more appropriate behavior. That comes through extensive and diligent training and modification. I would research BAT and contact a certified applied canine behaviorist in your area to help you address the problem.
Will that affect my other dog (German shepherd)?
Yes, likely. I would walk then separately. Your descriptions seems to suggest he is suffering from anxiety due to the introduction of the stimuli - dog- perhaps produced by her. The BAT work and a behaviorist could help you further to prevent this from developing into something inappropriate or unwanted.
How do I make her stop and become friendly again?
Realize that you may not achieve that goal. Likely you can with management and cover conditioning get her to the point where she may be able to be in a controlled setting without reacting to their presence. It is possible you could get her to be friendly again, that is possible but don't expect it. She has now reached maturity, all the characteristics of her breed and all the experiences she has had have had time to simmer and are expressing themselves in her current personality. I am not saying this is your fault, that you did anything wrong in raising her. Akitas are wonderful creatures but they can be challenging. Going forward, there will be a lot of work for both of you. I hope you reach your ultimate goal but do not be crestfallen if you don't. It is best to be realistic and take it a day at a time.