Personally I've found that joy cometh before a fall. Literally. My worst rollerblading accidents were when I was feeling joy (those hand guards do not protect against broken fingers). As such joy is not an emotion I actively cultivate, and when it happens I take it as reason to be wary.
In addition to surveys, there are interviews at regular intervals, so observations of the participants are also used as data points.
> There is a theory that I'd like to believe that everyone is equally happy over a period of time after basic necessities of life are satisfied.
Empirical evidence does not support this. There are such things as 'set points' of happiness:
* https://www.psychologytoday.com/ca/blog/meditation-for-moder...
Some folks have lower, and some higher, 'base levels' of happiness. It seems that some people are / can be 'inherently' happier than others.