There are a number of ways to configure redirects in the site.
Sometimes we change the name of an article but want its old URL to redirect to its new URL. For these types of redirects, we use redirect_from frontmatter. See /content#redirect_from for details.
Sometimes the canonical home of some content moves outside the help site. For these types of redirects, we add entries to /lib/redirects/external-sites.json.
We also have custom routing code that automatically creates redirects under the hood for things like moved Admin guide pages. This code lives in /lib/redirects/get-old-paths-from-permalink.js. All redirects for the site are compiled when the server starts by /lib/redirects/precompile.js.
See Links and image paths for info on how links and images are rewritten on the fly at page render time.
See Troubleshooting for info on viewing the redirects for any page.