Example of how to add block templates to post types in WordPress

block-template.php

<?php

function mytheme_block_templates( $args, $post_type ) {

  // Only add template to 'post' post type
  // Change for your post type: eg 'page', 'event', 'product'
  if ( 'post' == $post_type ) {

    // Optionally lock templates from further changes
    // Change to 'insert' to allow adding other blocks, but lock defined blocks
    $args['template_lock'] = 'all';

    // Set the template
    $args['template'] = [
      [
        // Example of including a core image block
        // Optional alignment setting        
        'core/image', [
          'align' => 'left',
        ]
      ],
      [
        // Example of including a core paragraph block        
        // Optional alignment placeholder setting         
        'core/paragraph', [
          'placeholder' => 'The only thing you can add',
          'align' => 'right',
        ]
      ],
      [
        // Example of including a custom block        
        // Optional placeholder setting        
        'custom/your-blocks', [
          'placeholder' => 'Custom placeholder',
        ]
      ]
    ];
  }

  return $args;

}
// Hook function into post type arguments filter
add_filter( 'register_post_type_args', 'mytheme_block_templates', 20, 2 );