Depression isn't sadness.
It's a generalized lack of emotional energy, where ordinary tasks like bathing or taking out the garbage are so exhausting that you inevitably perform the cost/benefit analysis: Is this worth doing? Where the answer is most certainly No.
It's like walking under water. The simplest gestures require unreasonable effort: you can force yourself to it, but it just isn't worth the expenditure. So that at its worst you spend months in bed, 'cos, What's the point in getting up? With filthy sheets, 'cos, Why wash them if they'll just get dirty again? Surrounded by garbage, 'cos, Why take that out when there'll just be more? Speaking to no-one, 'cos, OMG what could they want from you that you could possibly give?
Depression is crying constantly for no rational reason. If you stand up to walk to the bathroom you burst into tears, for no cause you can fathom, because you don't feel sad. It's more like feeling overwhelmed, where you're crying from the frustration of knowing you're unable to accomplish the things you're expected to.
Sadness, hopelessness, despair: those emotions don't apply. For one thing they'd require energy and you just don't have the juice. Depression is not caring, because caring implies effort and you've used up your supply of effort, somehow, sometime, probably long ago, probably so far in the distant past you can't remember what effort felt like.
Depression is about simple things being complicated, easy things being hard, the things everyone else does without thought requiring so much expenditure of planning and coordination that thinking about them makes you need sleep.
Depression is about seeing no reason for making the effort because the effort is exhausting and it brings nothing you care about.
So that you gain weight, lose friends, lose jobs, avoid the sunlight, avoid the world, without giving a fuck, with total certainty that the world returns the disinterest.