I’ve started making a horizontal mobile game, and it has just dawned on me that unity games made with a 16:9 aspect ration might not automatically play in a horizontal aspect ratio. Is this the case?
If you want it to start and stay in a specific orientation you can set that in Edit → Project Settings → Player → Android Settings (The buttons on the bottom) → Resolution and Presentation → Default Orientation
By default it is set to Auto-Rotation, which means it will display and switch however the user is holding the device (or how the user has locked the rotation of his device).