Restrict specific pages in your WordPress site for logged in users only.

my_restricted_pages_require_user_login.php

<?php
/**
 * Restrict specific pages in your WordPress site for logged in users only.
 *
 */
function my_restricted_pages_require_user_login() {
	global $current_user;

	// Allow logged in users.
	if ( is_user_logged_in() ) {
		return;
	}

	// Restrict all visitors from these pages.
	$restricted_pages = array(
		'private-page-1',
		'private-page-2',
	);
	
	if ( is_page( $restricted_pages ) ) {
		wp_redirect( home_url( 'wp-login.php?redirect_to=' . urlencode( $_SERVER['REQUEST_URI'] ) ) );
	} else {
		return;
	}
}
add_action( 'template_redirect', 'my_restricted_pages_require_user_login' );