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)
Anyway I would still like to have a choice of how to reply. I’m not used to use side swipes and I disable them all options in all apps by default. In some cases swipe down or up can be easily mistaken with side swipe, but using icon (or even menu of the message) seems so clean to use.
there is a still a choice, however, depending on the operating system, that will require and additional tap, discussion for that is at Items on main top menu vs items in 3 dot menu
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: