To provide a bit more detail then - you would setup your proxy with DNS entries "foo.example.com" as well as "bar.example.com" and whatever other sub-domains you want pointing to it. So your single IP address has multiple domain names.
Then your web browser connects to the proxy and makes a request to that server that looks like this:
GET / HTTP/1.1
Host: foo.example.com
nginx (or apache, or other reverse proxies) will then know that the request is specifically for "foo.example.com" even though they all point to the same computer. It then forwards the request to whatever you want on your own network and acts as a go-between between the browser and your service. This is often called something like host-based routing or virtual-hosts.
In this scenario the proxy is also the SSL endpoint and would be configured with HTTPS and a certificate that verifies that it is the source for foo.example.com, bar.example.com, etc.