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This can be YouTube, Hulu, Netflix, and many other apps. The placement of this icon varies by app, but you'll usually find it at the top-right corner of the app. Look for the rectangle with Wi-Fi waves at its bottom-left corner. This will open a pop-up window with a list of devices on your network you can connect to. This will connect the app on your phone to your TV. The selected video or song will play on the TV but not the phone, which means you can still use your phone while streaming.

Summary:
Open the app you want to cast to the Smart TV. Tap the cast icon in the app. Select the Smart TV. Select something to play.