1.The idea is that each Mat object has its own header, however the matrix may be shared between two instance of them by having their matrix pointers point to the same address. Moreover, the copy operators will only copy the headers and the pointer to the large matrix, not the data itself.
2.Sometimes you will want to copy the matrix itself too, so OpenCV provides the cv::Mat::clone() and cv::Mat::copyTo() functions.
Mat F = A.clone(); Mat G; A.copyTo(G);