How do I show a block with a unique QR for each node?

There is requirement where I need to Place a Block on node where each block will be having the Unique QR code.

I created an hard-coded QR code block with the following code.

public function build() {
  $path = '';
  $directory = "public://Images/QrCodes/";
  \Drupal::service('file_system')->prepareDirectory($directory, \Drupal\Core\File\FileSystemInterface::CREATE_DIRECTORY);
  $qrName = 'myQrcode';
  $uri = $directory . 'QR'. '.png'; // Generates a png image.
    
  $path =  \Drupal::service('file_system')->realpath($uri);
    
  \PHPQRCode\QRcode::png("www.google.com", $path, 'L', 4, 2);
  
  $relative_file_url =  \Drupal::service('file_url_generator')
    ->generateAbsoluteString($uri); 
    
  $qr_image = "<img src='{$relative_file_url}'/>";
      
  return [
    '#markup' => $qr_image,
  ];
}

The only issue is that I am not able to add the block to nodes nor to make the QR code dynamic.

It means that Each node will have a unique QR Code Block.

How to create a dynamic block inside hook_preprocess_node ?

Please Help

This is the original description of the task (now removed from the question):

Build the following:

• A Drupal content type to hold all their products
• A Drupal block that can be placed on any product page. The block, when placed on any product page, automatically shows the currently displayed product's App Purchase Link as a QR code, that the site visitors can scan using their mobile.

You can use the current route to get the node and add a unique block for each node:

/src/Plugin/Block/ExampleBlock.php:

<?php

namespace Drupal\mymodule\Plugin\Block;

use Drupal\Core\Block\BlockBase;

/**
 * Provides an example block.
 *
 * @Block(
 *   id = "mymodule_example",
 *   admin_label = @Translation("Example"),
 *   category = @Translation("mymodule")
 * )
 */
class ExampleBlock extends BlockBase {

  public function build() {
  
    $build = [];

    $node = \Drupal::routeMatch()->getParameter('node');
    if ($node instanceof \Drupal\node\NodeInterface) {
      // calculate $qr_image with $node
      $build = [
        '#markup' => $qr_image,
        '#cache' => ['tags' => $node->getCacheTags()],
      ];
    }

    $build['#cache']['contexts'] = ['route'];
    return $build;
  }
}

2. Version

Implementing the same block with a node context, as @leymann suggested:

/src/Plugin/Block/ExampleBlock.php:

<?php

namespace Drupal\mymodule\Plugin\Block;

use Drupal\Core\Block\BlockBase;

/**
 * Provides an example block.
 *
 * @Block(
 *   id = "mymodule_example",
 *   admin_label = @Translation("Example"),
 *   category = @Translation("mymodule"),
 *   context_definitions = {
 *     "node" = @ContextDefinition("entity:node")
 *   }
 * )
 */
class ExampleBlock extends BlockBase {

  public function build() {
    $node = $this->getContextValue('node');
    // calculate $qr_image with $node
    return [
      '#markup' => $qr_image,
    ];
  }
  
}

The advantage, you don't need to take care about caching, the context does this automatically. The disadvantage, you need to make the block dependent on a context when placing it in the block layout. Drupal is not yet able to map a block context automatically. You could, for example, add a visibility for the content type, which is probably a good idea anyway.

1. The simple answer is: if you don't want/can't use the https://www.drupal.org/project/twig_tweak - then just copy the code of the drupal_block Twig extension from that module to your own module.

2. If that is not the case - just simply implement hook_preprocess_node and put your block build code there or load the block and send it to a node template via $variables.

3. Use the layout builder module (in core) to place the blocks on the node pages.

The Barcodes module https://www.drupal.org/project/barcodes provides a block displaying a barcode of any type, including QR code. This task is as simple as enabling the module then configuring the block to display what you want. No coding necessary. Barcodes supports tokens so you can dynamically encode and display a product ID, a node ID, or any other information in your barcode block.