That's halfway correct - I'll try and break it down a bit further into the various parts.

Your subdomains are managed in using DNS - if you want to create or change a subdomain, that happens here. For each of your services, you'll create a type of DNS entry called an "A record", containing your service's full domain name, and the IP address of your reverse proxy (in this example, it is 10.0.0.1)

The DNS records would look like the following:

• plex.example.com, 10.0.0.1
• octoprint.example.com, 10.0.0.1
• transmission.example.com, 10.0.0.1

With these records created, typing any of these domains in a browser on your network will connect to your reverse proxy on port 80 (assuming we are not using HTTPS here). Your reverse proxy now needs to be set up to know how to respond to these requests coming in to the same port.

In the reverse proxy config, we tell it where the services are running and what port they're running on:

• plex.example.com is at server.example.com:32400
• octoprint.example.com is at server.example.com:8000
• transmission.example.com is at server.example.com:8888

Now when you type the domain names in the browser, your browser looks in DNS for the "A record" we created, and using the IP in that record it will then connect to the reverse proxy 10.0.0.1 at port 80. The reverse proxy looks at the domain name, and then connects you on to that service.

What we've done here is taken all 3 of those web-based services, and put them onto a the same port, 80, using the reverse proxy. As long as the reverse proxy sees a domain name it recognises from its config, it will know what service you want.

One thing to note though, reverse proxies only work with web-based services