The extension links into the Librivox api
https://librivox.org/
How the website works Help | LibriVox Help | free public domain audiobooks
The extension provides links to the chapterUrl, cover and message.
The App Inventor code uses Listviews to display the results.
Shorter explanation: Magic 
Here are some resources to help you learn to use the AI2 tools and understand how they work.
A very good way to learn App Inventor is to read the free Inventor's Manual here in the AI2 free online eBook App Inventor 2 Book: Create Your Own Android Apps ... the links are at the bottom of the Web page. The book 'teaches' users how to program with AI2 blocks.
There is a free programming course here Course In A Box and the aia files for the projects in the book are here: App Inventor 2 Book: Create Your Own Android Apps
How to do a lot of basic things with App Inventor are described here: How do you...? .
Also look here App inventor español. Offline. Tutorial. Ejemplos. Instalación. Códigos. Juegos. Curso gratis de App inventor. and here Tutorial Index | imagnity for more tutorials including Imagnity.com List Tutorial, Mirrored - @Saj
Learn about components Component Reference
and visit the Library The MIT App Inventor Library: Documentation & Support Help>Library on the MENU