I really like how if you visit https://invisioncommunity.com/4guides, you're automatically redirected to a specific article, https://invisioncommunity.com/4guides/welcome/invision-community-r7/

But how is it done? Is it just nothing more straightforward than a .htaccess redirect? If so, fine, I can do that myself. But I'm curious about whether it's achieved otherwise, say by a line of code on the page itself. If it is, what would such code look like?

For a listing template, you can just take the first record URL (if there is one) in the loop and forward there. 
{{\IPS\Output::i()->redirect($row->url(), '', 302);}}
That way, it is always automatic. 
Not sure if their index page uses something similar or a hardcoded forwarding. 

7 hours ago, opentype said:

Not sure if their index page uses something similar or a hardcoded forwarding. 

A mixture of both has worked for me on the index page:

{{\IPS\Output::i()->redirect('welcome', '', 302);}}

That redirects to the category with the slug welcome and your code in the listing template ensures that the first article in that category is displayed.