Is it even possible to build a profitable product on your own besides your job?

I wondered how much effort and work was behind the first really earned dollar from a product. Is it perhaps even a delusional idea to believe that you could even manage to generate any "passive income" at all, besides your job with only 8-16 hours per week? And would it even be easier to scale and make more profit than you thought, than to actually start making profits?

I am very much looking forward to hearing about your experiences!