Is this, like, "Next Post" and "Previous Post"? Or something else?

Yes

I think this needs to be handled by the Query block. You don't add navigation to a single post, you add it to a query result.

Take this blog post for instance https://riad.blog/2020/01/28/embrace-the-modularity/
it's not a query since we're showing a single post in a single.html/php and at the end, we have a "next/preview post" links.

We might consider that this is not a Core block (though that's a Core theme so we probably need it), but there's still a need for such block.

The context should still be provided by a Query block even if the Query block is implicitly included in the UI for a "single" template.

A post context has no notion of the list of posts it's in, so it cannot provide data for pagination UIs.

Mmm, I'm not sure that the context would come from a Query block or the implicit $wp_query of the single template. Specially considering you might want to display the prev / next post anywhere in the design (sidebar, footer, root with absolute position, etc).

Specially considering you might want to display the prev / next post anywhere in the design (sidebar, footer, root with absolute position, etc).

In those cases, where would it come from if not the implicit $wp_query?

Sorry, I meant to say but from the implicit $wp_query of the single template.