The standard copy tool of QCAD acts like the copy tool of other applications. It does not allow you to choose a reference point for your selection but instead computes the reference point automatically at the center of the selection. This is not always desirable as you most likely want to place the pasted entities at a precisely defined position. For this, QCAD offers and alternative, more CAD specific copy tool: Copy with Reference.
The selection is now on the QCAD internal clipboard and can be pasted into the same or a different drawing that is opened with the same running instance of QCAD. Note that if you quit the QCAD application, the contents of the clipboard is lost. The QCAD clipboard can only be accessed by QCAD itself. You cannot exchange drawing entities with other applications through copy and paste. Please use the export and import functionality of QCAD for this.