Send registration email to all admins

Better try something like this:

add_action( 'register_new_user', 'send_admin_registration_email', 99, 1 );
function send_admin_registration_email( $user_id ) {
    $user_info   = get_userdata( $user_id );
    $first_name  = $user_info->first_name;
    $last_name   = $user_info->last_name;
    $user_login  = $user_info->user_login;

    $subject="Family Medicine Residency Site Registration";
    
    $message="<p><b>Name:</b> " . esc_html( $first_name ) . ' ' . esc_html( $last_name ) . '</p>';
    $message .= '<p><b>Username:</b> ' . esc_html( $user_login ) . '</p>';
    $message .= '<p><a href="https://intranet/departments/residency/index.php/confirm-email/?id=' . esc_attr( $user_id ) . '">Click Here to confirm the registration.</a></p>';
    
    $admins     = get_users( array('role' => 'administrator') );
    $admin_emails = wp_list_pluck( $admins, 'user_email' );
    
    if ( ! empty( $admin_emails ) ) {
        wp_mail( $admin_emails, $subject, $message, array('Content-Type: text/html; charset=UTF-8') );
    }
}