HOW DO i MAKE MY WEBSITE PAGES TO RECOGNIZE LOGGED IN USERS

I am new to PHP and MYSQL, I need help to understand how to make my website pages recognize logged in users. i have the register and login system working. But I don't know how to make the web pages recognize users when they login.

i only know how to Simulate user login for testing using session_start();
$_SESSION['user_id'] = (int) 1;

But I want it to be Automatic and for multiple users, like social media website do.

Thanks for your generous assistance