We can connect the Survey123 to our Portal on premisses, but we miss the functionality from "Survey123.esri.com" with My survey's and all the "download" possibilities....is it there or coming??
The functionality you're asking about regarding Survey123 and how it works with Portal for ArcGIS are still in development and not currently available with Portal for ArcGIS at this time. This will NOT be part of the Portal for ArcGIS 10.4 release (coming in Q1 of 2016), but a later release.
Hope this helps,