Drupal

1. 1.robots.txt: replace the robots.txt in the Drupal web root (the folder with index.php) with the generated one, or install the RobotsTxt module to edit it from the admin (it requires deleting the physical file). On Composer-based sites, the drupal/core-composer-scaffold plugin overwrites robots.txt on core updates: exclude it or append your rules via file-mapping in composer.json.
2. 2.llms.txt: upload llms.txt to the same web root, next to index.php, so it is served at /llms.txt. The default .htaccess serves .txt files; on Nginx check that the location rules do not route it to Drupal.
3. 3.Structured data: the cleanest way is the Metatag module with Schema.org Metatag. Otherwise paste the content of json-ld.html into your theme's html.html.twig, right before </head> (or add it with hook_page_attachments in a custom module).
4. 4.Meta: with the Metatag module, update the front page title and description (Configuration > Search and metadata > Metatag) using the texts in meta-suggestions.html.
5. 5.Clear the caches (Configuration > Performance or drush cr) and re-scan the domain on AIVIS.

Download the bundle from your AIVIS report first, then re-scan the site to check the score went up.