After downloading the Flipbook Extension, open the package.

If you purchased Single Package :
You will see there is a file : 

Upload the files to your site using the Joomla Installer.

You can find the Joomla Installer in the admin area of your Joomla website, 
link : administrator/index.php?option=com_installer&view=install

You will receive a success message after successful installation.

Now you can start the installation of the extension.

Go to Plugins section and make a search for Flipbook
link: administrator/index.php?option=com_modules

Joomla PWA extension is compatible with Joomla 4 and Joomla 3

You can install the same package on both Joomla 3, Joomla 4and Joomla 5