Discover content is algorithmically ranked by what Google thinks a user would find most interesting. So the more relevant and interesting a piece of content is, the more likely it is to be shown in the Discover feed. There is, of course, a bit more to it than that.
Any content is eligible to appear in Discover results providing it meets the following two criteria:
The content is indexed by Google (you can check this using the URL inspection tool in Search Console, or by simply searching for the page in Google)
The content meets Google’s news and content policies