Since I came to Mac from Unix, I've done large complicated things mostly using the Terminal, which has a traditional Unix (Gnu and BSD) command interface, including the cp command. Using that with the find command it is possible to do what I'm describing, but it's a bit complicated, enough for me to never bother. I often write Tcl programs to do tricky things with regards to moving and copying files (and I now have a program intended for release called Collection Assistant which consolidates groups of files in arbitrary ways and eliminates duplicate file contents, when files are different content with same name they are renamed...why is it so hard to do THAT...)
"Moving" rather than "Copying" is assumed in the Mac drag-and-drop interface. I guess that makes sense. But how do I make it do copying? I was not able to find the answer in 10 minutes of googling. They gave other answers which are more complicated in my view. I was sure there was something easier.
So I went back to a more traditional way of finding things out on a computer. I just tried things. The Command key didn't work, but guess what, the Option key does! When you're holding the Option key, a plus symbol (+) appears when you hover over a folder in finder with a selected bunch of files. I'm sure people have told me this before also, but somehow it never stuck.
It's taken me 25 years using a Mac to discover that all you have to do to copy files rather than moving them is to hold down the Option key. It's actually very intuitive.
But why is it so hard to find out when googling for it?