Find out below.
Some note
To share your screen using FaceTime, first make sure both you and the person you're sharing with are using iOS 15.1 (on iPhone), iPadOS 15.1 (on iPad), or macOS 12.1 (on Mac) or later. You also need an Apple ID to use FaceTime.
Also, keep in mind that you won't be able to screen share content from apps that require a subscription or purchase account to view. (Apple recommends using SharePlay in this case).
In particular, the screen sharing feature in FaceTime will be very useful when you need help troubleshooting or want to present some information to others remotely. While sharing your screen, all incoming notifications on the device will be hidden for privacy. However, be careful if you have sensitive information on the screen, as the person on the other end can totally see it.
Share iPhone, iPad screen with FaceTime
To share the screen of your iPhone or iPad, first launch the FaceTime app and make a call to the person you want to share the screen with.
Once the connection is established, tap the 'Screen Sharing' button on the toolbar (it's a rectangle with a person in front of it). In the pop-up, tap 'Share My Screen'.
When screen sharing begins, FaceTime will be collapsed to a small window on the screen, and you can use your iPhone or iPad as usual to launch other apps. The person on the other side of the call will see what you're doing on the screen.
To stop screen sharing, reopen the FaceTime toolbar by tapping the video thumbnail. Then click the share screen button again.
When you're done with the FaceTime call, tap 'End'.
Share your Mac screen using FaceTime
To share your Mac's screen, you first open the FaceTime app and make a call to the person you want.
Once the connection is established, click the FaceTime icon in the menu bar at the top of the screen and click the share screen button, which looks like a rectangle with a person in front.
The menu will expand and you will have the choice to share a specific window or the entire screen. Click the option you want to use.
If you select 'Window', FaceTime will prompt you to click on the window you want to share. In case you choose 'Screen', the person on the other end of the call will see your entire Mac screen.
To stop screen sharing, click the FaceTime icon in the menu bar again and select 'Stop Sharing Screen'.
You can resume screen sharing at any time. When the call is done, open the FaceTime menu again and click the red 'X' button to disconnect.
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