The data is fetched from Google's CrUX API (specifically this API). This is the same data that Google exposes via pagespeed.web.dev or Google's search console. The tool itself does not capture or store any data.

Google PageSpeed displays the current Core Web Vitals. In contrast, this tool displays the Core Web Vitals for the last six months. The historical data allows us to identify when a certain degradation or improvement happened. This enables us to correlate any performance regression to the code, infra or other changes on our website.

For every Url visit via Google's Chrome, the browser sends anonymized core web vitals stats to Google. Google aggregates this data and makes it available via Google's CrUX. So, the data rendered by this tool is based on real users visiting the provided Url.