I am presently (February 2022) writing a book entitled "How I Sold 5 Million Books." This is about 5 million more books than most author's sell and was very difficult to do.
I did many things that other author's don't do to accomplish this. For example, I found famous authors in other countries and asked them if they would like to become my co-author for their country. They had to translate the book and change it enough so it would truly become their book as well. Because the end-product was good, and because they were famous, our book often became a bestseller in their countries.
I have never heard of another person doing this, and I have always wondered why.