Anyone can sign up and share example prompts, whether it's one they came up with or one they saw online (in which case they can give the source credit).
The submissions are then ranked the exact same way HackerNews articles are ranked on the homepage, following this algorithm: https://medium.com/hacking-and-gonzo/how-hacker-news-ranking... - in short, it's based on how many votes the prompt has received and how long since it was posted.