You’re going to get yourself into a whole heap of convoluted CSS mess if you concentrate on optimising for individual devices/screen sizes – there are way too many to account for now, and heaven knows what’s on the way in the future.
Instead, optimise for your content.
At its most basic level, resize your browser window, and when your content doesn’t fit into your layout, or your layout goes a bit hinky, set a media query breakpoint (tweakpoint) to fix your CSS within it.
It helps if you mostly avoid absolute positioning and work in relative units that will fluidly expand and contract (% and vw/vh, etc) between breakpoints too.
If you need more help with anything specific, you’ll need to provide some code or a sample page.