It does both. It initially goes to the bottom to show chronological order of comments. But if you use the gray reply button in a text box it attaches directly to that post also.
If you look in the bottom left corner of a post you may see in light gray text “Replies” with a down arrow, Click on that and you can see all the direct comments to that particular post.
Another technique is if you are browsing through a thread and see what seems like a random comment and you wonder what post it was in reply to look in the top right corner for a reply curled arrow with a members icon. Click on that and it will take you to the initial post that the current poster was replying to.
Hope this helps a bit.