I’m getting ready to publish my first game & I just wanted to know if unity automatically adjust to different & all screen resolutions for devices as in the background & sprites to fit the same for all devices or do we have to manually adjust it?
You should adjust your game elements and UI manually to fit the most common resolutions. This can be checked changing the aspect ratio of the game preview in the Editor:
In most cases the game will adjust itself to the resolution except the Unity UI. This guide should help you through the process of adjusting anchors and screen scale to fit every possible ratio.