The URL syntax is protocol-dependent. You find a detailed description in RFC 3986.
If you provide a URL without a leading protocol:// scheme, curl guesses what protocol you want. It then defaults to HTTP but assumes others based on often-used host name prefixes. For example, for host names starting with “ftp.” curl assumes you want FTP.
You can specify any amount of URLs on the command line. They are fetched in a sequential manner in the specified order unless you use --parallel. You can specify command line options and URLs mixed and in any order on the command line.
curl attempts to reuse connections when doing multiple transfers, so that getting many files from the same server do not use multiple connects and setup handshakes. This improves speed. Connection reuse can only be done for URLs specified for a single command line invocation and cannot be performed between separate curl runs.
Provide an IPv6 zone id in the URL with an escaped percentage sign. Like in
"http://[fe80::3%25eth0]/"
Everything provided on the command line that is not a command line option or its argument, curl assumes is a URL and treats it as such.