WordPress/wp-content/themes/twentynineteen/index.php

<?php
/**
 * The main template file
 *
 * This is the most generic template file in a WordPress theme
 * and one of the two required files for a theme (the other being style.css).
 * It is used to display a page when nothing more specific matches a query.
 * E.g., it puts together the home page when no home.php file exists.
 *
 * @link https://developer.wordpress.org/themes/basics/template-hierarchy/
 *
 * @package WordPress
 * @subpackage Twenty_Nineteen
 * @since 1.0.0
 */

get_header();
?>

	<section id="primary" class="content-area">
		<main id="main" class="site-main">

<?php
	if ( have_posts() ) {

		// Load posts loop.
		while ( have_posts() ) {
			the_post();
			get_template_part( 'template-parts/content/content' );
		}

		// Previous/next page navigation.
		twentynineteen_the_posts_navigation();

	} else {

		// If no content, include the "No posts found" template.
		get_template_part( 'template-parts/content/content', 'none' );

	}
?>
		</main><!-- .site-main -->
	</section><!-- .content-area -->

<?php
get_footer();