How do I reply to a specific message?

If you want to respond to previous comment in a Talk message, just press and hold on the comment, a drawer will open with options to react, reply to the message, copy post and flag as inappropriate.

Tap Reply and send your message like normal.

How do I react to a post?

You can react to any message in a Talk conversation by long-pressing on the comment. Press and hold, a drawer will open, enabling you to select a reaction.