What is an Enterprise Account and how do I find my URL?
In regards to "What is an Enterprise Account?"
Enterprise logins allows members of your organization to sign in to ArcGIS Online using the same logins they use to access your enterprise information systems. For example, the same sign-in you use to log into your computer in your organization could be the same sign-in you use to log into ArcGIS Online.
The advantage of setting up enterprise logins using this approach is that members do not need to create additional logins within the ArcGIS Online system; instead, they can use the login that is already set up within their enterprise system. When members sign in to ArcGIS Online, they enter their enterprise user name and password directly into your enterprise's login manager, also known as your enterprise identity provider. Upon verification of the user's login, the enterprise identity provider informs ArcGIS Online of the verified identity for the member who is logging in.
See this link for more info on how it can be set up; Set up enterprise logins—ArcGIS Online Help | ArcGIS
And for the second portion of your question "How do I find my URL"
There are various ways you can see your URL (the URL is only visible once you are logged in to your ArcGIS Online account).
1. Once logged in, check the web address bar at the top of your browser
2. Once logged in to ArcGIS Online, go to the My Organization page > Edit Settings > General > In the Organization Short Name section, you will see the Organization home page URL: http://xxxxxxxxxx
Hope this helps.
This gets a little harder to give a clear answer to since
With the ArcGIS 10.5 release, ArcGIS for Server became ArcGIS Enterprise. Learn about upgrading your deployment.
But I think how you are using it you are talking about an account within an Organizational ArcGIS Online site. Are in an agency or organization that might have an enterprise license for ArcGIS Online? Or are you using a free or developer ArcGIS Online site? I know I've seen a nice t chart that would show the differences (and will add if I find it), but this talks about the free account and have links to other info
Evaluate ArcGIS Online for Organizations | ArcGIS Online
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