When designing a vertical version of the mobile game UI, I wonder if the UI does not stretch, decrease, or break when turning the mobile device horizontally, and if it can be changed naturally to fit the horizontal version as the size of the widget, which widget or blue print is possible? like this
There is not a single widget that does all that automatically. Look into Canvas and Anchors. These allow you to attach UI elements to screen corners / centre + offset and keep things relative. Read up on widget Desired Size and how containers conform to / override it. Understanding how Fill works with lists will be crucial here, too.