Copy Minecraft Maps?

I wanted to copy a map to another known minecraft server and copy it so I can put it on my server. How does that work?

You ask the operator if you get the files of the map.

No the server is too well known. There's no mod

The mod you are looking for is called World Downloader, crazy I know, but on most servers you will not get the maps anyway and there are even servers that ban you instantly if you join them.