How can i make a login screen but the owner and the site admins are the only ones who are allowed to modify the username and password

I’m just wondering How do I make a login screen but the owner and the site admins are the only ones who are allowed to modify the username and password. I also want to make it so the site visitors can put in their login credentials on to my site that the site owner and admins set for them.

Can you elaborate

So basically i want to create a login system that follows the same principals as a regular login system, but the only difference is that the site admins and owner set the username and password for the members of my site