Building a livestreaming application with Twilio Live and Express involves creating a server-side application that handles both the streamer's and audience members' experiences. The application uses Node.js and Express for the server side and vanilla JavaScript on the client side. The streamer experience includes creating a Twilio Video Room, capturing audio and video streams, and sending them to an audience member through a PlayerStreamer. The audience experience involves connecting to a livestream using the Twilio Live Player SDK, playing the content in the browser, and displaying it in real-time. A temporary public URL is generated using ngrok for testing purposes, allowing users to share links with others to view or participate in the livestream.