Since iOS 14, it's possible to reply to a specific message in an iMessage conversation.
Just long-press on the desired message, then tap on "Reply" and write your reply.
That's it!
Your reply will appear at the bottom of the conversation with a link to the message you replied to.
And here will be a blue "1 reply" link underneath the message you replied to.
Just tap on it to see the reply.
It only works with iMessage (blue bubbles) not SMS (green bubbles).
Here's a quick demo:
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