As Lancifer said, Scale Outer will help adapt to various aspect ratios. For Android, a good starting place is an aspect ratio of 16:9, as this is the standard aspect ratio for Android phones as of 2010.
That being said, other phones with higher quality displays and curved screens will use a different aspect ratio, but using Scale Outer will fill the remainder of the screen nicely. I test on a Galaxy Note 9 18.5:9 but use a 16:9 aspect ratio with Scale Outer and it fills perfectly.
16:9 aspect ratios: 1024×576, 1152×648, 1280×720, 1366×768, 1600×900, 1920×1080, 2560×1440 and 3840×2160. If you use say 1366 x 768 you will see more of the background displayed etc compared to 1024 x 576. You will have to adjust your ratio/viewport to what suits your game.