How SharePoint getting domain user details before user profile synchronization

Hi Everyone,

I have a big query on SharePoint behind the screen, like how SharePoint getting a domain user details before User Profile services application starts.Scenario: Installed new SharePoint 2010 Farm and service application not yet configured then local administrator only has registered in Managed Account, but when i tried to create a new site collection, i can assign a domain user as a primary and secondary site collection administrator. How those details are knows SharePoint ?

i hope you all got my question .