Neither Apple nor Google does not advocate the use of a specific orientation for their devices as a default.
Material design only guides that layout should adjust to suit the screen size and orientation.
The iOS Human Interface Guidelines says:
If it’s essential that your app run in a single orientation, support both variants. An app that runs only in landscape mode should be
usable regardless of whether the user rotates the device left or
right. An app that runs only in portrait mode should rotate its
content 180 degrees when the user rotates the device 180
degrees—except on iPhone X, which doesn’t support upside-down portrait
mode. If your app doesn’t rotate automatically when someone holds the
device in the wrong orientation, they’ll know instinctively to rotate
it. You don’t need to tell them.
Customize your app’s response to rotation according to context. A game that lets people move a character by rotating the device, for
example, probably shouldn’t switch orientations during gameplay. It
could, however, display menus and intro sequences based on the current
orientation.
MORE TECHNICAL DEPTH:
Both iOS and Android use the readings of the accelerometer to define the current orientation if the auto-rotation option is on. If the auto-rotation is off, then user rotation is used instead.
On Android, the user rotation for the app can be set via USER_ROTATION variable. The possible values are:
USER_ROTATION: actual rotation, clockwise, 0 0°, 1 90°, 2 180°, 3 270°
It might mean that more preferred landscape rotation is 1, which is 90° clockwise.
On iOS, from Swift documentation (Swift is a programming language for iOS) user rotation values are listed in UIDeviceOrientation:
case landscapeLeft
The device is in landscape mode, with the device held upright and the home button on the right side.
case landscapeRight
The device is in landscape mode, with the device held upright and the home button on the left side.
Which may refer that more common orientation is landscapeLeft, because it has a lower value (3vs4).
