Hi,
If Portal and Server are behind the firewall and the Web Adaptor is forward facing on the web server configured to ArcGIS Server, and we are using Built-in users and Built-in groups on ArcGIS Server and ArcGIS for Portal and I install a new 10.5 Portal, do I need to do any of the https stuff and installing the second web adaptor for Portal?
See this link: About configuring portal authentication—Portal for ArcGIS (10.5) | ArcGIS Enterprise
Configuring built-in users and groups using the portal's identity store
No steps are necessary to configure the portal when using built-in users and groups; the portal is ready for built-in users and groups immediately after installing the software. If you are using enterprise users, see the following sections and related links for more information.