That’s not really correct, Discourse is a flat discussion system. You can expand replies but it’s a bonus, nice-to-have, optional interaction, not a primary interaction. The expected and typical way to read a topic is straight down, seeing all replies in chronological order.
If I press F12 in my browser, topic replies all look like this:
<div class="topic-post clearfix regular"><article id="post_6" data-post-id="364499" data-user-id="26120" class="boxed onscreen-post">
<div class="topic-post clearfix regular"><article id="post_7" data-post-id="364553" data-user-id="28717" class="boxed onscreen-post">
That’s two post replies, repeat over and over for more, etc.