Your app will need to subscribe to
reaction_added event. Once subscribed, you’ll receive this event each time a reaction is added to a message visible to the bot. This means, this you’ll receive the event for the channels where the bot is a member. Or direct messages sent to/by the bot.
If you specifically want to process events where the message was sent by the bot, look for
item_user property in the body of the event. You can compare it with the bot user ID to know if it was a reaction to a message originally sent by the bot.
Also, if you are using Node.js, consider using the official framework made by Slack – bolt. It makes a lot of things simpler.