You’re probably right that your question might be better suited for another post, but I’ll answer it anyway…
The idea of focusing on layout versus device width or resolution is that you start with a layout that would look good on the smallest devices and scale that up until the layout or some elements of it look terrible – this is your breakpoint. Adjust the layout and continue sizing up until your layout fails again and you have another breakpoint.
The idea is that your breakpoints will (most likely) not correspond to a specific device width or resolution but will optimize the layout no matter the device, giving you a responsive site no matter what new devices come out
