How to react on WhatsApp Chat
Hold down for a long time on the message you want to react to: the entire line will be selected, and above it will appear a list of emojis, graphically larger than the message itself, or your reactions
Choose the reaction you want, press on it, and it will be “sticked” to the selected message.
How WhatsApp reactions work on Android
To remove a reaction already sent simply plug it on it with your finger: it will disappear for both you and the other chat participants.
To change a reaction already sent you have to repeat exactly the same seen above to enter it.
Each message can only have one reaction for each user.
If you reacted to an ephemeral message, the reaction will appear with the message itself.
Reactions cannot be hidden, meaning you cannot enable/disable them from settings.
However, you can turn on notifications for reactions or not, both for groups and for single chats.