There are 2 types of REPLIES in HO.
A. NORMAL REPLY
The blue button under all the posts is a reply to the whole thread, to the first post, to be exact.
B. Nested Replies
If you want a kind of nesting structure, when one replies to a post, by using the reply button under the post (see example below)
Under that post, you will see X Replies on the left.
Click X Replies, you will see all the related replies to that post.
I think it’s the best this system can do. Note that this structure works only on computer, tablets, it won’t work on phones (unless you use the function of requesting Desktop Website in your mobile browser)