The Influencer Search API is available as part of RapidAPI's API marketplace, and it provides a way for developers to find authority content written by influencers and build an engagement with them through social media. To use this API, you need to sign up for a free RapidAPI developer account, subscribe to the Influencer Search API, and test the API endpoint. The API returns a JSON response containing an array of profile information for each influencer. A demo web app can be built using Ruby on Rails to demonstrate how to call the API and display the results in a user-friendly format. The app includes features such as a search form, a table displaying the influencer's information, and a spinner that indicates when the "Find Influencers" button is being clicked.