Compatibility of the game on different screen sizes

Hi everyone! I’m developing a Mobile 2D game in Unity but i have a big problem and question.
It can synchronize gameobjects such as text and buttons in the canvas according to the screen size. But it can’t synchronize gameobjects that are not in the canvas. How does everyone sync their game between different devices? Do they use code to fix gaps on screen edges, scale game objects to the screen, and other things like that? If they don’t use it, how do they do it?

usually ppl accept that the game will show more of the screen on mobile, theres not much to do except prepare for the camera to display extra space on the sides / or to cut them a lot if its on portrait view