AGOL > Enable Esri Access vs. My Esri > Set Permissions:
Let's take "E-Learning" as the example:
...First of all, if these two Accounts are Linked, is it by the 'Username', the 'E-Mail' or 'combination'?
If Accounts are Linked, which of these, or both, need to be set to get E-Learning access?
If the Accounts are separate (NOT Linked), which one, or both?
And what specifically about E-Learning is being accessed? ...Because someone with NO Account can access it.
These are not two accounts.
This is one Esri Account that you use to access both ArcGIS Online and My Esri. Your Esri Account is identifiied by its unique username. In ArcGIS Online the administrator can configure the account to have Esri access. The same Esri Account can be granted permissions when it is connected to an organization in My Esri.
In My Esri, when an Esri Account is granted "Access e-Learning" permissions, that user now has full access to self-paced e-Learning on the Esri Training website, as long as that organization has a qualifying product with current maintenance.
You can read more about this in the Training section of the Common Questions on My Esri at https://myqa.esri.com/#/questions