For new authors, we often recommend that you start publishing directly to Amazon and take advantage of the KDP Select program so that you can use some of Amazon's promotional tools and list your books in Kindle Unlimited. Learning one ecosystem is great when you are starting out to avoid overwhelm, but what about once you are comfortable with Amazon and are ready to sell your books wide in other retailers?
You could upload your book to other retailers directly, but that's a new learning curve for each one you come across. Another option is to use a distributor, which allows you to upload your book once, and then sell the book in multiple stores without having to know how each one individually works.
In this training, we are going to talk about Draft2Digital, as well as number of free tools that they offer that you can make use of even if you never pay them a single dime.