Make unity camera appear all screen width

For this you need to define what width you want your world to be (Let's say 800 units long). After that you need to get the current aspect ratio of the camera. Calculate the camera height based on the world width and aspect ratio of the camera, then apply to the orthographic size.

 float aspectRatio = (float)Screen.width / (float)Screen.height;
 float cameraHeight = WORLD_WIDTH / aspectRatio;
 
 camera.orthographicSize = cameraHeight/2f;

The reason we use half the desired camera height for the orthographicSize is because that's what the orthographicSize actually is:

orthographicSize

This should leave everything centered and lock the camera to your world width while different devices show more or less vertical height. If you want your camera to always be at the bottom of the level you'll need to adjust the camera's vertical position.

Hope this helps.