Rather than copy extensive page layout or page configuration code and then modify what you want to change, in the Magento system, you only need to create an extending layout file that contains the changes you want.
Changing layout files is one of the two possible ways to customize page layout in Magento 2 (the second way is altering templates). To change the page wireframe, modify the page layout files; all other customizations are performed in the page configuration or generic layout files.
Magento 2 application implements the Model-view-controller architecture pattern; meaning, the Magento 2 software is architected into layers, including the view layer.