although pretty well known from other messengers and apps, this is not an obvious option and worth mentioning here:
to reply to a message on android or iOS, tap the message and immediately move the finger to the right - when you see the reply icon, you can release the finger from the screen:
(the video shows a mouse cursor for easier video recording - imagine, this is your finger, moving on the bubble to the right, and then release)
alternatively, one can long tap the message and select “Reply” from the menu (iOS) or from the three-dot-menu (android)