To reply to a single text message (formerly “quote”), tap and hold it until the context menu appears. There, tap the arrow icon (“Reply”), and the message will show up above the text input field. Alternatively, you can swipe right on the message.
Then, compose and send your message like you usually would. In the chat, the message you’re replying to is indented and prepended to your new message so that it’s evident what you are referring to.