There should be Freeze option to be available in MS Teams, so that while presenting our screen, we can freeze our current window & do whatever we want without letting viewers.
Viewers only can see the window which we freezed and once we are ready with our new content to show, we can unfreeze that presentation to let viewers see our current live screen.
Its like a Freeze option we have in most of the Projectors we use in face to face meetings or presentation.