They show up above the posts, below the navbar

A random one is chosen to show up every time someone visits the page. You can check out https://programming.dev for some examples of them (usually itll be an http status code or a quote)

Instances either put announcements there (e.g. future downtime), core info (e.g. links to frontends, donate link) or random sentences. Can only be one of the options out of those three though unless you want to delete and remake things a bunch of times to swap between them if you want everyone to see the downtime message