Game Development For Different Screen Resolutions

Hello, I've been wondering what does unity do when designing for multiple resolutions, does it scale your game to fit other sizes? I have created a background which is 16:10 aspect ratio, when I switch to a 480x320 resolution then the background becomes smaller and the Camera background is shown on the sides. How is this logical that when a big sized background looks good on a big resolution but becomes small on a smaller resolution, does is adjust my background? (By the way, using orthographic Camera 3.6 size) What can I do to make it right in every resolution? to what resolution should I design?
Another thing is when designing a game in which the player is in the corner of the screen, in a smaller resolution it disappears. what can I do to make my game look good at every possible resolution?