Strange behavior for Finder when copying folder

Dear guys, I'm new to mac, so please bear with me.
I noticed that whenever I copy a folder to a destination with the same folder name, the finder will ask me whether to replace the file or not. When I choose replace, it will delete the destination folder and then it will copy the folder to the destination.
to confirm this, I put a folder named test on /Users, and I also put the test folder on Desktop.
On my /Users/test I create a test1.txt, and on my desktop/test I create a test2.txt, then I copy from /Users/test to /Desktop, when the finder asked the replace question, I answered yes, and it's true the test folder on the desktop was erased, then it copied from the /Users/test. So in my desktop test folder, instead of 2 files (test1.txt, and test2.txt), there's only 1 file (test1.txt)
Is it normal? if not how to correct it? if yes how to append the file to the existing folder?
Sorry for my English
Thanks

Dear MacboobsPro,
I think either you misunderstand my meaning (because of my english) or you're completely wrong.
What I mean is whether you're using linux, windows , you can copy a folder on one harddisk to another harddisk with the same folder name, but you didn't erase the content of the folder, but you're adding the content from folder1 to folder1 (located on another drive)