What is the difference between a mental illness and a symptom?

First off, I’d like to say I do not mean to sound disrespectful here at all. I am purely curious and want to understand how this works.

For an example, what is the difference between laziness and someone with depression? Is laziness also caused by a lack of dopamine, or is it something else? What’s the difference between the symptom in a depressed person and the symptom in someone without depression?