I am new to Affinity Photo, and have watched the tutorials and read the program Help file in detail.  Must have missed something though . . .   After making a selection in a photo, how do I modify just the selection?  Do I create a layer?  If so, how?  This seems so basic, but I can't find where the instructions are nohow to do it.  The video tutorial on Making Selections and Refining them, just don't explain what to do with them.  What have I missed?  Any assistance on how to proceed would be