How to write a RSS feed plugin…

I am sure that this is available online but for the life of me I cannot figure out how to word it to google to find it…

I want to write a plugin that will produce a RSS feed (Don’t ask – it’s mainly for my own edification on how to write this). I have the code that produces that feed. I can follow basically everything for rolling it into a function with only one little thing escaping me :

How do I hook into wordpress so that when a URL is called it responds? I mean I can respond if I dropped the function code into a file and plopped that file in wp-content/themese/theme/rss_feed.php but I want to have it respond when I call “https://www.blah.com/rss_feed”.

I figure it must be a hook I add but I cannot figure out which one it is…