You're seeing just the function get_reactions, go back to Nostrum.Api module for more information.
Link to this function

get_reactions(channel_id, message_id, emoji)

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Gets all users who reacted with an emoji.

This endpoint requires the VIEW_CHANNEL and READ_MESSAGE_HISTORY permissions.

If successful, returns {:ok, users}. Otherwise, returns Nostrum.Api.error/0.

See create_reaction/3 for similar examples.