What is the Chromecast built-in application, how does it work?

According to the introduction of Google LLC, Chromecast built-in is a Tools app on the Android platform. There is currently a 1.44.194651 version released on Feb 10, 2020, you can see the details below.

Google Cast is a technology that enables multi-screen experiences and lets a user send and control content like video from a small computing device like a phone, tablet, or laptop to a large display device like a television.

This app includes Chromecast built-in for Android TV.

Available and pre-installed only on Google approved Android TV devices.