Short summary for no patient
now user can access hub open data site through this URL
https://data.victorvilleca.gov
when user type above request in browser address bar, it hit city's DNS server, city DNS server will forward user to Arcgis.com.
Arcgis.com resolve this URL to the true real hub site URL as this
https://opengis-victorville.opendata.arcgis.com
So 2 URL is identical, but with different domain, on is city domain, as xxxx.victorvilleca.gov
original is xxx.arcgis.com
Long story
Every arcgis hub site you created will have the default site URL as xxx.hub.arcgis.com
you can customize the subdomain, XXX section of it, but you can't change or remove ESRI's domain hub.arcgis.com part.
What if I want my open data site name use our own domain, for example data.victorvilleca.gov, instead of use arcgis.com domain?
First, you need IT to create a CName record, which is a subdomain forward on your domain setting,
then you test it here, you get validated.
after save, now user can access hub open data site through this URL
https://data.victorvilleca.gov
when user type above request in browser address bar, it hit city's DNS server, city DNS server will forward user to Arcgis.com.
Arcgis.com resolve this URL to the true real hub site URL as this
https://opengis-victorville.opendata.arcgis.com
So 2 URL is identical, but with different domain, on is city domain, as xxxx.victorvilleca.gov
original is xxx.arcgis.com