The get_block_file_template WordPress PHP filter allows you to modify the block template object after it has been fetched from the theme file.
Usage
add_filter('get_block_file_template', 'your_custom_function', 10, 3);
function your_custom_function($block_template, $id, $template_type) {
    // Your custom code here
    return $block_template;
}
Parameters
- $block_template(WP_Block_Template|null) – The found block template, or null if there is none.
- $id(string) – Template unique identifier (example: theme_slug//template_slug).
- $template_type(string) – Template type: ‘wp_template’ or ‘wp_template_part’.
More information
See WordPress Developer Resources: get_block_file_template
Examples
Add custom template for a specific block
Modify the block template if the template identifier matches a specific value.
add_filter('get_block_file_template', 'custom_block_template', 10, 3);
function custom_block_template($block_template, $id, $template_type) {
    if ($id === 'your-theme-slug//your-template-slug') {
        // Modify the $block_template object here
    }
    return $block_template;
}
Add a fallback template
Provide a fallback template if the $block_template is null.
add_filter('get_block_file_template', 'fallback_block_template', 10, 3);
function fallback_block_template($block_template, $id, $template_type) {
    if ($block_template === null) {
        // Set a fallback $block_template object here
    }
    return $block_template;
}
Change the template based on post type
Modify the block template based on the current post type.
add_filter('get_block_file_template', 'change_template_for_post_type', 10, 3);
function change_template_for_post_type($block_template, $id, $template_type) {
    global $post;
    if ($post->post_type === 'your_post_type') {
        // Modify the $block_template object here
    }
    return $block_template;
}
Modify template for a specific template type
Modify the block template for a specific template type, such as ‘wp_template_part’.
add_filter('get_block_file_template', 'modify_template_type', 10, 3);
function modify_template_type($block_template, $id, $template_type) {
    if ($template_type === 'wp_template_part') {
        // Modify the $block_template object here
    }
    return $block_template;
}
Change template based on user role
Modify the block template based on the current user’s role.
add_filter('get_block_file_template', 'change_template_for_user_role', 10, 3);
function change_template_for_user_role($block_template, $id, $template_type) {
    if (current_user_can('editor')) {
        // Modify the $block_template object for editors
    }
    return $block_template;
}