I don't understand forms with respect to log in and registration

Question

What are the different codes for registration and log in and how do they relate (do I have to link the registration page in the log in page...

After creating a registration form, what happens after a user have registered (does it actually start working after the website have been uploaded to a web server) ..

Answer 1

It doesn't matter if you do two pages or not. But both should send there data to the same backend so you can check if the credentials are correct and so on.

If you regisger and press the post button you send those data to an server which does something with it. That isn't visible for any user.
To prevent that one person registers two times you can check in your backend if the user already exists and forward him to your login page or throw an error or something.