How to do layout files

interior-clean.php

<?php
namespace CLIENT;

class Interior {

	public static function init() {

		add_action( 'wp', function () {
			if ( ! is_front_page() ) {
				self::hook_wordpress();
			}
		} );

	}

	public static function hook_wordpress() {
		add_action( 'before_wrapper_div', [ __CLASS__, 'get_header_int' ], 20 );
		add_action( 'after_content_div', [ __CLASS__, 'get_sidebar' ], 20 );
	}

	/**
	 * Gets the interior header if we're not on the home page.
	 */
	public static function get_header_int() {
		get_template_part( 'partials/header-int' );
	}

	/**
	 * Gets the sidebar.
	 */
	public static function get_sidebar() {
		get_sidebar();
	}
}

Interior::init();

interior-messy.php

<?php

add_action( 'before_wrapper_div', 'client_get_header_int', 20 );

function client_get_header_int() {

	if ( ! is_front_page() ) {
		get_template_part( 'partials/header-int' );
	}  
}

add_action( 'after_content_div', 'client_interior_get_sidebar', 20 );

function client_interior_get_sidebar() {

	if ( ! is_front_page() ) {
		get_sidebar();
	}
} );