如何查找已扩展到哪个主模板以查看刀片？

In register.blade view file I have following script

@extends(subdomain(), [
    'bodyExtraClasses' => 'page-login layout-full',
    'pageExtraClasses' => 'vertical-align text-center'
])

I am not sure this subdomain() extends which files into register.blade? Also there are two different register url but having same register template.

I know how to extend the master layout but this is new for me. I did some search about it but could not found good guideline or I am still fail to understand it. Also in this project we are using sub domain.

I have found the function in the helper (functions.php)

function subdomain()
{
        return str_before(Route::getCurrentRoute()->getDomain(), '.' . env('domain'));
}