Reactions appear under the message, with a number when more people chose the same emoji. Move your mouse over a reaction to see who reacted. Click your reaction (it has a coloured border) to take it back, or someone else's to add the same one.
Reactions are shared between wiply.net and the Narendi app: a reaction added in one place shows in the other.
Good to know: the text of your messages is encrypted, but, as in every Matrix app, reactions are not: our server can see which emoji was added to a message, never what the message says.