When it comes to ebooks, Amazon has been and continues to be the undisputed king of ebook sales. You’ll sell more downloads there than anywhere else on the Internet. At least, I guess you still can. There are actually books I’ve sold through other retailers that are not selling at all on Amazon. I have no idea why. In any case, you can create an account to publish on their digital text platform here: https://dtp.amazon.com/mn/signin.
When you sign up with Amazon, you have a choice between joining KDP Select (publishing only on Amazon) or staying completely indie. Choosing to be Amazon exclusive (available on Amazon only) or uploading your work to other publishing platforms. The exclusivity period runs for 90 days at a time, with the option to re-up as often or not as you choose.
However, don’t limit yourself to Amazon. When I first self-published, Amazon was the only game in town. Now authors and readers have other options.
You can publish directly to different retailers or use a distribution service.