If you want to reply to a particular response in a long threat, you can also use the “permalink” feature to the right side of the “Flag as…” link in the response. (It’s an ephemeral link that only appears when you hover the cursor over it, but it will give you a permanent link to “that particular response,” which you can then paste into a response using the editing tools.
So here is a link to an earlier response (from @Stinley) in this particular thread.
You’ll get the hang of it soon enough.