How to limit Posts on Posts page to a specific Category in WordPress. https://sridharkatakam.com/how-to-limit-posts-on-posts-page-to-a-specific-category-in-wordpress/

functions.php

add_action( 'pre_get_posts', 'sk_show_posts_from_a_category_posts_page' );
/**
 * Show Posts from a specific category on Posts page
 *
 * @author Bill Erickson
 * @author Sridhar Katakam
 * @link http://www.billerickson.net/customize-the-wordpress-query/
 * @param object $query data
 *
 */
function sk_show_posts_from_a_category_posts_page( $query ) {

	if( $query->is_main_query() && !is_admin() && is_home() ) {
		$query->set( 'category_name', 'category-1' ); // Replace "category-1" with your category slug
	}

}