How does ftpd service access the FTP protocol to understand how to transfer files?

This question applies not only to the FTP protocol, but also to other protocols in general. We call FTP a protocol, but speaking literally, FTP has a source code. When an ftpd service is running on a server, how does this service access and make use of the FTP source code? Is the source code embedded into the ftpd service? Many thanks and cheers.