How does Http_proxy environment variable work?

What is Http_proxy vs Https_proxy?

I was reading a book called Advanced Networking Programming, in which a chapter was dedicated to explaining the differences between Http and https proxy.

The author says that Http proxy can be used for Https communication (and vice versa), but I've never seen it in practice. For example, I have a client which communicates with a webserver via Http proxy. It doesn't work if the client is set to communicate with the webserver over https, but it works fine when the client communicates with the webserver over http.

Is this true? And how can I make my client communicate with the webserver over https? The author says that Http proxy can be used for Https communication (and vice versa), but I've never seen it in practice. For example, I have a client which communicates with a webserver via Http proxy. Is this true?
No, it's not true. The book is not as well written as it could be.

As for how you can make your client communicate with the webserver over https, you can use some of the protocols available to communicate with https proxies. One option is to use the WebSocket protocol. Another option is to use SOCKS.