The landing page is light on details, from the about page in the browser it is based on Microsoft Edge WebView2 [1] and a blog post from last year [2] says
> It’s not a “fork” of any other browser code; all the code, from tab and bookmark management to our new tab page to our password manager, is written by our own engineers. For web page rendering, the browser uses the underlying operating system rendering API.
They may be gearing up to go out of beta as I got a notification from their landing page for the first time today
[1] https://developer.microsoft.com/en-us/microsoft-edge/webview...
[2] https://spreadprivacy.com/windows-browser-open-beta/
