If a proxy receives the tcp packet, then the destination ip address and port are that of the proxy. In that case, I imagine the only way a proxy can still resolve the intended destination is either through routing the source ip address/port or through the host field in the http header. Is this correct?
In both Fiddler and Charles http proxies, I noticed that the tool accepts both http and https connections through a single port that you can specify. What do these tools do to tell the difference between the two types of connections?