The Survey123 Website is not available as a disconnected installation for ArcGIS Enterprise which is a real shame.
For organizations with very stringent security requirements which only allow access to their Enterprise through VPN gateways and do not allow the use of ArcGIS Online, this means that Survey123 is not available to those Enterprises.
Survey123 is additionally the only Esri solution I know of that is designed this way. Business Analyst, Ops Dashboard, Insights, Workforce, GeoPlanner, AppStudio, Collector, Explorer, Maps for Office, Hub as well as all of the Esri Solution and Web App Builder Templates are fully available to Enterprises that do not connect to ArcGIS Online. Survey123 should be no different. It's crazy that Survey123 could be as mature as it is at this point and still not be productized for installation behind the firewall.