Tim Hardwick, MacRumors:

Once ‌FaceTime‌ screen sharing has started, you can navigate to any app that you want to share with the callers. A sharing icon will remain in the top-left corner of the screen to indicate that ‌FaceTime‌ screen sharing is active, and you can tap it to reveal the ‌FaceTime‌ control panel.

Another delightful nugget coming with iOS 15, especially useful for folks who are tech support for their families.