A variety of sources, not just web scraping. Including DNS records, DNS traffic and more. All collected by us.
As for trends, while we dont have something like Builtwith trends, you can easily collect all the data yourself via the API see how its trending using your favorite BI tool.
And yes we are missing some JS frameworks like nextjs and react. We chose to focus more on paid products instead. Builtwith (and its competitors like Whatruns, Wappalyzer and Similartech) is missing data on a lot of backoffice/backend products which we have data for