At the bottom of the screen you'll see a button labeled Cast my screen -- tap it. Then, tap Cast screen on the prompt that appears to begin casting whatever is on your phone or tablet onto your TV.

Screen mirroring allows you to display your phone screen on a larger display like a TV in real-time. There are different methods to mirror Android to TV using Chromecast, Roku, or PC/Mac with ...

OBS, the streaming software, sees that mirrored screen on your computer and uses it as a source for your stream, like a camera. So, people watching your stream, see what's happening on your phone!