I’m not sure what you mean.
If you proxy to
localhost:8080, it will contact the local host (the machine it’s hosted on) on port 8080 and send a HTTP request. If it gets a HTTP response back (which it will, if Apache is serving a site on port 8080), it sends that response onwards to the client.
The fact that it’s Apache, or whether or not Apache is hosting a website, doesn’t really matter to Caddy. It just connects, grabs a HTTP response, and gives it back.