It is possible to install ArcGIS Enterprise on a server (named "A") inside a LAN, install the web adaptor on a public-facing web server (named "WebServer"), and reach ArcGIS services using https://example.com/arcgis.
Is it possible to install Portal in a similar manner such that the server running Portal is not directly reachable?
I saw a lot of documentation about how the Web Adaptor for Portal requires a fully qualified domain name but that doesn't exist for servers that aren't public-facing.
For instance, http://www.esri.com/videos/watch?videoid=3640&isLegacy=true&title=configuring-arcgis-web-adaptor-wit... shows a Portal URL of https://husker.esri.com:7443 at the 07:42 mark. Does that mean that if I typed https://husker.esri.com:7443 into my browser's address bar, I would see the default Portal home page and my network traffic would start bouncing around ESRI's internal network?
Yes, it's possible to configure Portal in the same way that you configure Server in your example. The Web Adaptor could be on a public web server and sending traffic to your backend Portal in your internal network. There's no difference between Portal and Server in that regard. Since you're not within the esri.com network, you won't be able to reach husker.esri.com.