I can confirm Siteuser comes from user information hidden list.
Users are added to the User Information List when:
The specified user is granted permissions individually within a site collection
The specified user is added to a person/group field value anywhere in the site collection
The user is named specifically in a workflow anywhere in the site collection
The user is added to a SharePoint group as an individual (i.e. not when added as a member of a domain group)
The user is granted permissions anywhere in the site collection via a domain group membership AND subsequently visits the site (if the user is granted access via a domain group membership but never visits the content they were granted access to, they are not added to the user info list)
The user is passed into the SPWeb.EnsureUser() method
"Everyone Except external" is a security group instead of sharepoint group. you can find it in siteuser instead of sitegroup.