The .mobi file is actually two files in one. One is used by more recent eReaders such as the Fire, PaperWhite, and Kindle Android apps. The other is used by older Kindle devices. After your book is delivered to Amazon they create a proprietary 3rd file type called an .azk file. This is the file delivered to Apple Books users with the Kindle app on their device.
If you’re checking your .mobi proof on an iPad or iPhone using the Amazon app, we strongly suggest that you check the ePub version in Apple Books- this will give you a much closer approximation of the true appearance of your final Kindle formatted file.
The only way to see how the eBook from Amazon will appear in an iOS device running the Kindle app is to purchase it directly from Amazon after it’s available for sale.