On a PC, you can normally just double click the downloaded zip file, and you will find the files inside. If a PDF is included, you can read this directly on your PC via Adobe Acrobat or similar. With an epub eBook, the Aquile reader from the Microsoft store is modern, fast and very easy to use.
iPads and iPhones have the functionality to open a Zip file in the Files app on the device. When you are on an iPad or iPhone, you will be given the option to download and save the file in your Files area on the iPad. If you then open the Files App, you can then tap on the downloaded file and the books will be automatically extracted into a folder on your iPad. Then simply tap the book you want to read, and it will open in the Books App.
Hope this helps!
