EDIT: I saw your edit to the OP. You're looking for the email, but if you take a look at the <d:PrincipalType>, its value is 4, which corresponds to a Security Group. Security Groups in AD don't always have email addresses, so I assume that in your example, the principal that you're looking at just doesn't have an email address.

See here: https://docs.microsoft.com/en-us/previous-versions/office/sharepoint-csom/ee541430(v=office.15)

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By ID, what exactly are you looking for? The <d:Id> node you underlined corresponds to the user's ID in the hidden user list. Email appears to be blank for this user, though <d:Email> is where you would see their email address if it was set in SharePoint's User Profile Service.

If you're trying to code something to get the ID, you'll first need to determine which of the ID nodes you need, and then load the XML output of the REST call into an XML object and traverse the XML to get to that node.