Here is a lots of ESRI hosted arcgis rest service api with organization-iD but don't know the owner name.
ESRI hosted arcgis online json file:
If you know Organization-ID
https://www.arcgis.com/sharing/rest/portals/<Organization-ID>?f=json
If you know your-organization-short-name
https://<your-organization-short-name>.maps.arcgis.com/sharing/rest/portals/self?f=json
By Knowing Organization-ID
8Fll56KQwYKXAi7m
You can get arcgis online portal JSON file
https://www.arcgis.com/sharing/rest/portals/8Fll56KQwYKXAi7m?f=json
urlKey is <your-organization-short-name>
name is their esri hosted portal name, usually tell you who is owner.
Knowing organization-short-name
You know their esri hosted portal, which is arcgis online is
<your-organization-short-name>.maps.arcgis.com
victorville is organization-short-name https://victorville.maps.arcgis.com/home/index.html
You also can find their self hosted enterprise portal URL
gis1.victorvilleca.gov is your-government-domain
portal is your portal-web-adaptor
https://gis1.victorvilleca.gov/portal/home/
Let's test this theory !
https://transparentgov.net/json2tree/esri/vendor/mappingsupport/list.html
Is this Organization-ID z6hI6KRjKHvhNO0r really is PG&E pacific gas ?
https://www.arcgis.com/sharing/rest/portals/z6hI6KRjKHvhNO0r?f=json
urlKey tells your organization-short-name is SCE2
https://sce2.maps.arcgis.com/sharing/rest/portals/self?f=json
Obviously sce.com is their domain.
Arcgis online home page is
https://<your-organization-short-name>.maps.arcgis.com
https://sce2.maps.arcgis.com/home/index.html
Now you know, Pacific Gas is not the real owner, the real owner is SCE.com, southern california edison