This shows how to create a custom post type and assign it a block template. This would likely go in your functions.php file.

custom-post-with-block-template.php

<?php

function mytheme_create_post_type() {
  register_post_type( 'mytheme_event',
    [
      'labels' => [
        'name' => __( 'Events' ),
        'singular_name' => __( 'Event' )
      ],
      'public' => true,
      'show_in_rest' => true,
       // Optionally lock templates from further changes
       // Change to 'insert' to allow adding other blocks, but lock defined blocks
       'template_lock'] = 'all';
        // Set block template
       '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',
          ]
        ]
      ]
    ]
  );
}
add_action( 'init', 'mytheme_create_post_type' );