Almost nothing, by design. The tools on this site run entirely in your browser. The numbers you type or the files you drop are processed on your machine and never sent to a server. There are no accounts, no signup walls, and we set no cookies.
For traffic counts we use Cloudflare Web Analytics, which is cookieless and doesn't profile visitors. It tells us aggregate things like how many people visited a page and from which country, and nothing about who you are.
Two standard exceptions, in the interest of honesty: the site is hosted on Cloudflare Pages, which like every host keeps routine server logs, and fonts load from Google Fonts, so your browser makes a request to Google's servers when a page loads. Neither involves anything you type into the tools.
Questions? Ask directly: Sebastian on LinkedIn ↗.