question about user login authentication process, and new plugin

I have installed the 2.2. What's the user login authentication process? I have read \modules\core\UserLogin.inc, and find there is a iteration of GalleryValidationPlugin to call each validation plugin's method performValidation. But where the concrete plugin located? I just find a captcha validation plugin, which seems not like the one check username/password. Or is there some way to do the validation?

What's more, how to create a plugin for different authentication when user login? Because I want to validate user login in other DB not G2's.

Any information is appreciated.

Thanks,
Amo