In the publication it is measured via smartphone app at random times in the day (with respect to the participants sleeping schedules).

> After providing informed consent, participants completed an intake survey, which included demographic questions as well as two measures of life satisfaction, as detailed below. The participants were next asked to indicate the times at which they typically woke up and went to sleep, and how many times during the day they wished to report on their experiences (default = 3). A computer algorithm then divided each participant’s day into a number of intervals equal to the number of desired reports, and a random time was chosen within each interval. New random times were generated each day, and the times were independently randomized for each participant. At each of these times, the participants were signaled via a notification on their smartphone, asking them to respond to a variety of questions about their experiences at the moment just before the signal. The experienced well-being question was asked in every survey. Other questions unrelated to the present investigation were also asked. The participants received notifications requesting a report until they chose to discontinue participation. If 50 samples had been collected, reporting stopped for 6 mo or until the participant requested that it be restarted.

The paper cited by OP (whose comment appears to be shadow banned) cites two surveys:

1. "450,000 responses to the Gallup-Healthways Well-Being Index" reported by Kahneman and Deaton (2010).
2. Data from trackyourhappiness.org as reported by Killingsworth (2021).