"Happiness/contentment" is effectively a "balancing thermostat" of our motivation, a functional aspect that is used by our bodies to regulate our behavior and thus, if it functions properly (as opposed to certain diseases e.g. anhedonia in clinical depression) its long term average will be pretty much the same no matter how well you do, it's almost orthogonal to any metric of actual wellbeing. For example, studies show both winning excessive amounts of money in lotteries and sustaining major life-changing injuries (e.g. losing limbs or causing other disabilities) do not correlate with happiness in the long-term. Happiness effectively reflects (a) recent short-term changes to wellbeing; (b) momentary expectations or worries about future wellbeing; and (c) innate baseline happiness. It's not a reflection of how well someone is living; in essence, people living long-term in a literal gulag may easily have on average the same overall happiness/contentment as living in a nice first world upper-middle-class environment.
The word "happiness" refers both to the temporary emotion and a long-term state of being satisfied with your life. Nowadays the long term state is often called "life satisfaction" in research.
Someone in a gulag would have low life satisfaction.
That study isn't the final word though. Other studies have found that winning the lottery increases life satisfaction, while still others have found that they don't.
