> I don't understand what the numbers in the lower corner mean
If you tap on the numbers in the lower corner, you can cycle through various options including % read. My personal preference is for time left in chapter.
Some Amazon Kindle books DO have "real" page numbers. It's an additional feature that must be supported. The problem of page numbers on reflowable text is much more difficult than it may appear.
Not totally trivial, sure, but I have a hard time believing that it's difficult. Maybe for complex documents, but just plain text? Not really. I use Moonreader on Android, it shows page numbers with no trouble.
Ok, well it has been available for zero books I've read on Kindle, and 100% of books I've read on moon reader. Whatever the challenge might be, other people have figured it out and Amazon hasn't. It's embarrassing.