Gutenberg: Prefix The Content with specified Blocks.

functions.php

<?php

/**
 * Prefix the content of Post Type Page with a set of Blocks.
 * @see https://wordpress.stackexchange.com/a/323469/218062
 */
function prefix_the_content_with_blocks( $args, $post_type ) {
	if ( 'page' === $post_type ) {
		$args['template'] = array(
			array(
				'core/group',
				array(
					'style' => array(
						'color' => array(
							'background' => '#eee',
						),
					),
				),
				array(
					array(
						'yoast-seo/breadcrumbs',
					),
					array(
						'core/heading',
						array(
							'placeholder' => __( 'Add an page title', 'textdomain' ),
						),
					),
				),
			),
		);
	}
	return $args;
}
add_filter( 'register_post_type_args', 'prefix_the_content_with_blocks', 10, 2 );