How to go about integrating feature into posts

[therealjr]

Originally my plan was to embed posts from other websites (subreddit, twitter, etc) in a post through a separate field that the user would then provide the link for. However what I think might be better is to automatically embed posts that are linked through the comment field. My question is, what files are pertinent to this task?

[DangerOnTheRanger]

If I understand you correctly, you're trying to modify the way the imageboard posts themselves are parsed. CONTRIBUTING.md has a high-level overview of what you'll need to modify and/or add. You'll want to make an extension that looks for links to sites you want to embed, and then inserts the appropriate HTML to embed the content.