Samsung Galaxy Note9 - Turn Screen Rotation On / Off
3D Accelerometer Rotation is a feature that allows flexibility in how the content is viewed on the screen. Simply turn the device from vertical to horizontal (or horizontal to vertical). The device senses the change in orientation and automatically adjusts to fit the screen.
Camera, album, music player, videos, web browser and many other apps take advantage of this feature.
Swipe down on the Status bar (at the top).
The image below is an example.
Swipe down from the top of the display to expand the quick settings menu.