How to use member login from EE - sessions and cookies

Hi there, 1st of…not an extension/module developer here just attempting….

How would I go about to using EE’s authentication method to integrate an application that can pull ‘member’ info from EE tables? Sample code for integration I have to work with is:

/**  
 * @return the user ID of the logged in user
 */
function getUserID() {
    $userid = 0;

    if (!empty($_SESSION['userid'])) {
        $userid = $_SESSION['userid'];
    }

    return $userid;
}

/**

What is relation with ‘exp_sessions’ table to cookies and sessions? From light-years ago my understanding was that sessions are not stored in database…but EE seems to do so. So how to go about tapping into EE authentication?