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!

How does Screen Mirroring contribute to streaming Android to OBS? Imagine your phone screen is a TV. Screen mirroring is like using a really long cable to show that TV on your computer screen.

One UI 6.1.1 removes the need to tap “start now” every time you want to stream an app from your Galaxy device to your Windows PC.