Odoo website, Creating a signup page for external users
User Signup is a standard feature provided by Odoo, and it seems that you already found it.
The database selector shows because you have several PostgresSSQL databases.
The easiest way is to set a filter that limits it to the one you want:
start the server with the option --dbfilter=^MYDB$, where MYDBis the database name.
User data is stored both in res.userand res.partner: the user specific data, such as login and password, are stored in res.user. Other data, such as the Name is stored in a related res.partner record.
How can I create a signup page in odoo website. The auth_signup module seems to do the job (according to their description). I don't know how to utilize it.

In the signup page there shouldn't be database selector
Where should I store the user data(including password); res.users or res.partner