Example of how to customise the output of a core block (since WP 5.8). Planned for integration to a Hello Roots Theme.

PostTitle.php

<?php

namespace SayHello\Theme\Block;

/**
 * Customisation of the Core Post Title block
 * Find the original core rendering at e.g. wp-includes/blocks/post-title.php
 *
 * @author Mark Howells-Mead <mark@sayhello.ch>
 */
class PostTitle
{

	public function run()
	{
		remove_action('init', 'register_block_core_post_title');
		add_action('init', [$this, 'registerBlockCorePostTitle']);
	}

	/**
	 * Server-side rendering of the `core/post-title` block.
	 *
	 * @package WordPress
	 */

	/**
	 * Renders the `core/post-title` block on the server.
	 *
	 * @param array    $attributes Block attributes.
	 * @param string   $content    Block default content.
	 * @param WP_Block $block      Block instance.
	 *
	 * @return string Returns the filtered post title for the current post wrapped inside "h1" tags.
	 */
	public function renderBlockCorePostTitle($attributes, $content, $block)
	{
		if (!isset($block->context['postId'])) {
			return '';
		}

		$post_ID          = $block->context['postId'];
		$tag_name         = 'h2';
		$align_class_name = empty($attributes['textAlign']) ? '' : "has-text-align-{$attributes['textAlign']}";

		if (isset($attributes['level'])) {
			$tag_name = 0 === $attributes['level'] ? 'p' : 'h' . $attributes['level'];
		}

		$title = get_the_title($post_ID);
		if (isset($attributes['isLink']) && $attributes['isLink']) {
			$title = sprintf('<a href="%1s" target="%2s" rel="%3s">%4s</a>', get_the_permalink($post_ID), $attributes['linkTarget'], $attributes['rel'], $title);
		}
		$wrapper_attributes = get_block_wrapper_attributes(array('class' => $align_class_name));

		return sprintf(
			'<%1$s %2$s>%3$s</%1$s>',
			$tag_name,
			$wrapper_attributes,
			$title
		);
	}

	/**
	 * Registers the `core/post-title` block on the server.
	 */
	public function registerBlockCorePostTitle()
	{
		register_block_type_from_metadata(
			ABSPATH . WPINC . '/blocks/post-title',
			array(
				'render_callback' => [$this, 'renderBlockCorePostTitle'],
			)
		);
	}
}