Smashwords

If you are a writer of fiction or non-fiction ebooks, the best way to self-publish your books is through Smashwords

Basically you upload your book to Smashwords, they give you a unique ISBN number for your book, and the book will then get submitted to Smashwords's partners - Apple Books, Barnes & Noble, Scribd, Kobo and many more, as well as being listed on the Smashwords site where people can buy it directly.

If you get sales from any of these platforms, you get paid your royalties via Smashwords. If you want your book listed on Apple Books, going through Smashwords is the easiest option.