Hi Enzo, I thought I had already answered your query. I'm sorry that you seem so far to have found the material you discovered on the web to be unsatisfactory. The whole reasoning betwen the use of antidepressant drugs ( and indeed, the drugs used for other psychiatric disorders ) is that they manage, in various ways, to correct these chemical imbalances. For instance, when there is a relative lack of a particular chemical which helps to transmit messages / impulses, between nerves, one type of antidepressant may slow down the way in which the chemical is de-activated or metabolized, so that the smaller amount will be present for longer, and increase the effective concentration where it matters.
Remember, that though these chemicals are vital to the normal functioning of the body, the actual amounts an concentrations in any specific micro-area are tiny.
The drugs help to correct the various chemical changes that have been part of the development of these illnesses, at first on a temporary basis, but after one has been taking them for a while, the body may make various changes that help to make these corrections more permanent. The delay of around 2 weeks before an antidepressant shows its full benefits is related to these gradual changes in the brain chemistry.