Curl is a command line tool to transfer data from or to a server, using one of the supported protocols (HTTP, HTTPS, FTP, FTPS, TFTP, DICT, TELNET, LDAP or FILE).
Linked server is a SQL Server feature that enables customers to read data and execute commands on remote databases servers outside of the SQL Server instance.
That value is copied into the SQL query by the server, based on what the user typed in, so the actual command that's sent to the SQL database would look something like this, where username equals philbin.