What psswd is stored in the db when user registers with FB or Gmail?

Hello,

If a user register/sign in with their facebook I see that wordpress creates a username and password for them in the database, but doesn't send an email with the registration credentials - am I missing a setting somewhere?

I am sure the password stored (encrypted) is NOT their facebook password since i do not believe this type of info will be passed on by facebook, so what is this password.

Now what if the user want to sign in using their wordpress user/pass on the site (wordpress - not face book although they originally signed up with facebook) - Of course the same question applies to Gmail account access.

Thanks for your help.