GUIs for different Screensizes, Resolutions, DPI´s

[Flaming Moe]

Hello there,

what is the approach to create GUI´s / Apps for different devices with different
Screensizes, Resolutions, DPI´s, width/height proportions and what descissions have to be done? For example one could be if the app is intended to be controlled by finger (big buttons, slider etc) or by a pen (small buttons etc.)
I know it is possible to read the pixels from the graphics driver. So it would be possible to feed this information to the constructor for every widget. But that would mean hell a lot of coding for every different screen, no?

So, what is a good approad to this and what compromises i have to do?

best regards!!