Restrict your WordPress site for logged in users only.

my_require_user_login.php

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

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

	// Allow all visitors to view these public pages.
	$okay_pages = array(
		'welcome',
	);
	if ( is_page( $okay_pages ) ) {
		return;
	}

	// Allow all visitors to view the homepage.
	if ( is_front_page() ) {
		return;
	}

	// Allow all visitors to view these Theme My Login pages.
	$tml_pages = array( 'login', 'lostpassword', 'resetpass', 'logout', );
	if ( is_page( $tml_pages ) ) {
		return;
	}

	// Allow all visitors to view URLs with the word login in it.
	if ( strpos( $_SERVER['REQUEST_URI'], 'login' ) !== false ) {
		return;
	}

	// Redirect everyone else to the login page.
	wp_redirect( home_url( 'wp-login.php?redirect_to=' . urlencode( $_SERVER['REQUEST_URI'] ) ) );
}
add_action( 'template_redirect', 'my_require_user_login' );

This recipe is included in the blog post on "You don’t need a membership plugin." at Paid Memberships Pro here: https://www.paidmembershipspro.com/you-do-not-need-membership-plugin/