There’s 200+ different viruses that cause the common cold. When you get a sore throat or runny nose you don’t consider those to be the disease, you know there is an underlying cause.
Do you think Depression is similar to that? Do you think it is a symptom of some other disease or diseases?
Since there are distinct subtypes (unipolar, bipolar, melancholic, atypical, psychotic, catatonic, etc), there’s probably different mechanisms underlying each one.
These are clinical diagnoses but IMO “depression” has a myriad of variances. Everyone’s experience is of course personal. If someone is clinically depressed they’re probably experiencing a bunch of different feelings caused by various emotional and physical circumstances.