Learning design by Copying someones design is a good idea to sharpen your design skills. But it also helps of you try to understand whats the design all about. For example, if you view a website daily and try to understand whats the website about, whats the concept behind the design if any. It will help you hone your skills. The other stuff you are talking about like best way to design for multiple device sizes, resolutions, etc are all easy to understand and will not take much off your time. Even if you look at some open source files you will understand the basics.
Learning design by Copying someones design is a good idea to sharpen your design skills. But it also helps of you try to understand whats the design all about. For example, if you view a website daily and try to understand whats the website about, whats the concept behind the design if any. It will help you hone your skills. The other stuff you are talking about like best way to design for multiple device sizes, resolutions, etc are all easy to understand and will not take much off your time. Even if you look at some open source files you will understand the basics.