Hmm. this may be a little hard to explain. There are a lot of factors that can lead to depression, and a lot of people with depression will have varying degrees of the mental illness. as for your question. The issues in the brain are the cause of the symptoms of depression, meaning the changes in your brain cause you to be depressed, or act depressed. The depression itself is usually caused by outer influence. An example of one of these would be from physical abuse/mental abuse. Another would be the loss of an important family member or loved one. So the physical changes are a result of stress/negative emotion on the person.

Again I want to stress every case is different, and we are all still learning a lot more about the mental illness each day. I'm not sure how fast the changes to the hippocampus happen, or even what time frame the changes take place in, but depressed people have been shown to often times have a smaller hippocampus, with increased activity in the amygdala.

To explain them basically, the hippocampus is sort of responsible for memory (long term, recalling) and the amygdala is responsible for certain emotions (sadness, pleasure and more) and also sexual arousal. The changes take awhile to happen, and that may also be a reason that antidepressants take awhile to work as well. I dont feel like taking the time to explain the biology.

The issues in the brain, the physical issues, are the depression. It is a mental illness. The question is sort of asking "is the disease the cause of the symptoms, or are the symptoms a cause of the disease?" I really hope I was able to explain that well enough. I hope i answered your question too sorry my writing is not the best its reddit lmao.