Arcgis online, or AGOL is esri hosted enterprise portal.
These portal URL always have same pattern:
<your-organization-short-name>.maps.arcgis.com
victorville is organization-short-name
https://victorville.maps.arcgis.com/home/index.html

How to find out this organization's self hosted Enterprise portal ????
ESRI deployed portal meta json is always:
https://<your-organization-short-name>.maps.arcgis.com/sharing/rest/portals/self?f=json
https://victorville.maps.arcgis.com/sharing/rest/portals/self?f=json

Finding 1:
allowedRedirectUris has self hosted portal URL, in this case is https://gis1.victorvilleca.gov/portal
authorizedCrossOriginDomains, also have self hosted URL who was used by third party vendor
In this case:
energovgis is for tyler software licensing software
sedaru is for water utility software
Finding 2:
Organization-ID is ID field, this case is 8Fll56KQwYKXAi7m

With Organization-ID you know their ESRI hosted arcgis server URL is
https://services[0 to 9].arcgis.com/<Organization ID>/arcgis/rest/services
0 - 9 you have to find out youself,
In this case, it is 3.
https://services3.arcgis.com/8Fll56KQwYKXAi7m/arcgis/rest/services

Finding 3:
Organization-short-name is urlKey field, this case is victorville
ESRI hosted portal URL always have same pattern:
<your-organization-short-name>.maps.arcgis.com
victorville is organization-short-name
https://victorville.maps.arcgis.com/home/index.html

Finding 4:
their gis admin email, if you have question, ask this email.
In this case :
mailto:yzhang@victorvilleca.gov
Finding 5:
Because of this portal is ESRI hosted, so ESRI disabled its self page. see below
https://victorville.maps.arcgis.com/sharing/rest/portals/self

If this is a self hosted portal, it has details of info, now use the self hosted portal URL you just found previouly to build portal home directory.
self hosted portal home directory is always:
https://[your-organization-domain]/portal-web-adaptor/sharing/rest
In this case, it is
https://gis1.victorvilleca.gov/portal/sharing/rest
click portals

Click self

You can find arcgis online link again, now you see, self hosted portal and ESRI hosted portal (AGOL) are linked together.
If you know one of them, you know the counter-part by the method I list here.
https://gis1.victorvilleca.gov/portal/sharing/rest/portals/self
