The .bsp isn't converted to .res it's a simple text file.
A .res file is telling the client there are additional resources that need to be downloaded from server.
It specifies the type, path and name of the resources.
As for the sv_downloadurl, it depends.
It points to an external download mirror in order to (usually) provide faster download speed and reduce bandwidth usage on the server.
So if you have set up an external webspace with good download speeds, you can have the client downloading the files from there, too.
If a file is missing on the webspace but available on the server, clients will still be able to download it there.
If you're using a fast dl mirror, it is a good idea to compress big files for faster download.
Source is using the bz2 format for this. It's available e.g. in programms like 7zip.
Just put the map.bsp.bz2 into the corresponding folder on the webspace.
Otherwise the game server will create a ztmp itself and send this to the client, but at the cost of server bandwidth.