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.

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!

While you use streaming services on your Fire TV stick, you may want to cast videos stored on your phone or have friends share from their phones.