How were thresholds per metric defined?

To create the three buckets, being green for good experience, orange for needing improvement and red for poor experience, Google looked at their very own CrUX data as well as user experience researches to come up with a threshold or bucket distribution. They also analyzed what the percentage of different amount of (in case of the LCP) seconds was. See also Defining the Core Web Vitals on web.dev website.