did Session Created and Stored UserId in session after Users Logged In??

hi all,
in some threads ,Good helper Told me that when the User Logged In, the UserId will not Stored In Session,
but i have test it and Type Session.Abanadon();  when the User Logged In,
then the User Will not be Authorized , this is Mean That The Session is Used here and Storing UserId in session !!
Can Any one tell me because im confused now !!
did Session Created and Stored UserId in session after Users Logged In??
if the answer Yes whats the Name of Session???
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