Create a WordPress post whose content can be included in other posts via a shortcode.

wordpress_shared_blocks.php

<?php

class sharedBlocks {

	public static function setup() {

		add_action( 'init', [ __CLASS__, 'register_post_type' ], 0 );

		add_shortcode( 'shared', [ __CLASS__, 'do_shortcode' ] );

	}

	public static function register_post_type() {

		register_post_type( 'shared', [
			'supports'            => [ 'title', 'editor' ],
			'show_in_rest'        => true,
			'taxonomies'          => [],
			'hierarchical'        => false,
			'public'              => true,
			'show_ui'             => true,
			'show_in_menu'        => true,
			'show_in_nav_menus'   => true,
			'show_in_admin_bar'   => true,
			'menu_position'       => 20,
			'menu_icon'           => 'dashicons-slides',
			'can_export'          => true,
			'has_archive'         => false,
			'exclude_from_search' => true,
			'publicly_queryable'  => false,
			'capability_type'     => 'post',
			'labels'              => [
				'name'                => 'Shared Block Containers',
				'singular_name'       => 'Shared Block Container',
				'menu_name'           => 'Shared',
				'parent_item_colon'   => 'Parent Shared Block Container:',
				'all_items'           => 'All Shared Block Containers',
				'view_item'           => 'View Shared Block Containers',
				'add_new_item'        => 'Add New Shared Block Container',
				'add_new'             => 'New Shared Block Container',
				'edit_item'           => 'Edit Shared Block Container',
				'update_item'         => 'Update Shared Block Container',
				'search_items'        => 'Search Shared Block Containers',
				'not_found'           => 'No Shared Block Containers Found',
				'not_found_in_trash'  => 'No Shared Block Containers Found in Trash',
			],
		]);

	}

	function do_shortcode( $atts, $content = '' ) {

		// ATTRIBUTES
		extract( shortcode_atts([
			'id' => 0
		], $atts ));

		// Abort if the ID is empty or matches the current post.
		if ( empty($id) || (int) $id == get_the_ID() ) return; 

		$shared_post = get_post( (int) $id );

		$content = apply_filters( 'the_content', $shared_post->post_content );

		return $content;

	}

}