Once that’s done, open the FaceTime app and start a call (select someone from the list of recent calls or tap New FaceTime and add a contact via the plus icon or by typing their name in the text box). Next, hit the screen-sharing button, which looks like a silhouette of a person next to a rectangle, followed by Share My Screen. If you’re in an app that supports SharePlay, like Apple Music or Apple TV, you may instead see a share content button, which looks like a person standing in front of some WiFi waves.
Source : https://www.popsci.com/diy/facetime-screen-sharing/
