The App blocks is a feature within the Theme Updater & Backups app's Theme sections that can be added to the theme's Products page template.
This section is only available to Theme sections Pro subscribers in the Theme Updater & Backups app.
If your theme does not already include the section, install the app then subscribe to the Theme sections Pro to add this section to your storefront.
To access it, open your Apps within the Shopify admin and click on Theme Sections:
Full access to all sections and blocks are available once you subscribe to the Theme sections plan:
Once you are subscribed, you will find several Theme sections and App blocks.
App blocks will have a "View details" button, while Theme sections will show an "Install" button:
After clicking View details, confirm that you are seeing the "Ready to use in your theme" message on the right side of the page, or click on the Subscribe & unlock all sections button to unlock the block.
Select the theme, and click Customize theme to start adding the block.
Using the App blocks
App blocks are only available within the Products page template.
Hover over the existing blocks in the template to Add block and click on the Apps tab. From the options, select the block: