How do I build apk that works on galaxy s8 aspect ratio (18.5/9)?

The way I solved t$$anonymous$$s issue for me is setting the canvas to scale to screen width/height, and setting a base resolution. It should automatically scale according to your screen size. also if you are trying to scale the game camera to the screen somet$$anonymous$$ng like t$$anonymous$$s may help

     public float distance = 10;
     public float goDepth = 1;
     Vector3 v3ViewPort;
     Vector3 v3BottomLeft;
     Vector3 v3TopRight;
 
     // Use t$$anonymous$$s for initialization
     void Awake () {
         distance -= (goDepth * 0.5f);
      
         v3ViewPort.Set(0, 0, distance);
         v3BottomLeft = Camera.main.ViewportToWorldPoint(v3ViewPort);
         v3ViewPort.Set(0.75f, 0.75f, distance);
         v3TopRight = Camera.main.ViewportToWorldPoint(v3ViewPort);
 
         transform.localScale = new Vector3(v3BottomLeft.x - v3TopRight.x, 
         v3BottomLeft.y - v3TopRight.y, goDepth);
         transform.Rotate(new Vector3(0,0,180)); 
     }

just play around with the values until you find somet$$anonymous$$ng that works for you