Vote for the stats you most want to see covered in the 2021 chapter on performance. Don't see one you like? Propose a new one with a new issue, and label it "proposed stat". Be clear about the specific data. Feel free to comment on issues with caveats, questions, etc.
To better understand what data is available, check out the 2020 chapter on Methodology. Generally speaking, we have access to HTTPArchive, CrUX, and Lighthouse data for about 7.5 million websites.
The app was created by Lea Verou and used for the 2020 Web Almanac CSS chapter. It's built on a tool called Mavo which asks for access to your repos to log in. You do not have to log in. You can use the links on the voting page to navigate to an issue and add a 👍 to the first comment in the issue. This is how the votes are tallied.