If you have iTunes (some iphones don’t have it anymore), then do this:

using a laptop or a desktop computer (Mac or PC, doesn’t matter), download the ZIP folders at the links I am pasting below.

Once a folder’s been downloaded, open the ZIP folder or “unzip it” (this either will happen automatically when you open it, or you can right-click on the folder and choose “extract all,” which is the same thing)

You’ll now see that the ZIP folder contains several MP3 audio files, all the individual sections/chapters of the book. Put those MP3 files into your iTunes library. There are several ways to do that, and they are listed at https://www.wikihow.com/Add-MP3-to-iTunes

(Optional: you can rename the files to make them easier to find if you have lots of audiobooks, or if you don’t prefer our chapter-listing system).

Then, “synch” your iPad, iPhone, or iPod with the iTunes account on the laptop or desktop. Apple gives instructions for it here: http://support.apple.com/kb/ht1386

Now all the MP3s will be on your mobile device and you can listen to them anywhere.

If you do NOT have iTunes on your phone, do this:
There is a reputable, free App that a lot of people have started using to open ZIP folders on their iPad, and also to play video, look at PDFs, listen to audio, etc. It is a file management app called DOCUMENTS. It can play music files. You don’t need to use Apple Music to play the music files.

https://apps.apple.com/us/app/documents-5-file-manager-pdf-reader-and-browser/id364901807

Here are the directions: https://helpspot.readdle.com/documents/index.php. It’s useful for learning how to download files, share them, etc.
Section 1.8 gives more info about playing music files.