It's a small website that you can set as the home page in your Steam overlay browser settings. It allows you to register bookmarks on a per-game basis, and search the web with the search engine of your choice.
All your Steam Bookmarks data (your settings and your bookmarks) is exclusively stored in your browser, using localStorage.
Steam Bookmarks is open-source: https://github.com/tanguyMichardiere/steam-bookmarks - it's your typical Next.js + TailwindCSS app, the most interesting file is probably src/api/player-summary.ts, in which I implemented a non-trivial cache with Redis to avoid hitting the Steam Web API rate limit.
Cheers!