All the other mentioned protocols to speak with the proxy are clear text protocols, HTTP and the SOCKS versions. Using those methods could allow someone to eavesdrop on your traffic the local network where you or the proxy reside. Because over the connection between curl and the proxy, the data is sent in the clear.
One solution for that is to use a HTTPS proxy, speaking HTTPS to the proxy, which then establishes a secure and encrypted connection that is safe from easy surveillance.
When an HTTPS proxy is specified, the default port used on that host will be 443.
In all other ways, HTTPS proxies work like HTTP proxies.