So I wrote a book and here's what I found out about self publishing

As the title says, I just finished and published a book (1 of 3). I had visions of uploading my book for sale all across the universe, but was surprised to find out a lot of things that others may be interested in:

First of all, Amazon rules the world. Kindle direct publishing isn't perfect but it's user friendly and makes it easy for you to self publish.
Amazon takes 35%, but you can get more if you join the Kindle Select ( I think that's what it's called) and only sell on Kindle.
Kindle apps are available on all platforms, and you can even order a book via your Echo and it will also read it to you, as do the Kindles.

Barnes and Noble not only discontinued the Nook, but they are also shuttering their digital publishing in a few months.

Google has stopped taking submissions to the Play book store because of rampant copy write infringement. People were literally just copying books that were already published, changing the cover and author and republishing them as their own. It's been down for almost a year, I'm assuming they're cleaning it up and will open it again soon.
Google Shutters Its Play Books Publisher Portal in Order to "Improve Its Content Management Capabilities" | The Digital Reader

Apple doesn't really want your business unless you already own Apple products. You have to own a Mac to even be allowed to publish to the book store, or you have to go through a partner company which charges an average of $300 to publish it for you.

So basically if you're looking to self publish, Amazon is pretty much ruler of the digital publishing world and your best friend.