Each service runs/listens on its own port, including the proxy (typically 80/443). When you connect to the proxy using its port, it will look at the domain name you used and proxy your connection to the port for the service that name is setup for.
So when you go to expose these to the network/internet, you only have to expose the port the proxy listens to and the clients only ever use that port regardless of how many services/domains you host.