Are you asking what Discourse does to allow the site to be searched by Google? If so, the best answer that I know of is in this post: Beginners guide to SEO with Discourse. There are additional details in the other posts in that topic.
One thing to note is that topic mentions the Discourse Sitemap plugin. As far as I know, search engines have no trouble indexing a Discourse site even when it doesn’t have a sitemap. The main thing to work on is creating quality content on your site that people want to read.