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Zen dropped some knowledge, i'll try to expand on what I can.
To be safe, keep track of your royalties for the year (amazon might send you an end of year summary, but I can't verify that). Then, you can either report the income (if its tiny) in schedule E of your 1040. However, if you're at the level of wanting to establish your writing efforts as a business entity, there's some stuff in schedule C to fill out. A tax accountant is the easiest way to untangle all that as Zen mentioned, but an accounting software will usually guide you through the process. If you're raking in sales, then there may be some expenses you can deduct.
Amazon KDP will let you put a pseudonym as the author.
Ah the production process:
The internet is full of information on how to turn your word document into a book. Briefly, there are three main components that are usually outsourced by authors: editing, typesetting, and cover design. Those three things and some sundry efforts will get a book in your hands. Editing costs range from $0.02 to $0.10 per word depending on editor and amount of work needed to be done. Typesetting varies, but is usually $2-500 (ballparking here). And then a few hundred to the price of an engagement ring will get you from a basic cover to the kinda art you see on a beer can. Craft beer. From the hole in the wall place that doesn't have any seating and is only open one day a week because the rest of their time is spent drawing up one-of-a-kind can labels. Each one is a masterpiece. When you gravity chug whatever beer might be in there and crush the can on your head, god kills a kitten.
After that, amazon can function as your distributor. Essentially, they will provide you a link to sell your book. Expect zero spontaneous sales from amazon as they will in no way promote your book. No one will ever see it unless you are actively promoting it on another platform (social media, your own web site, or some real life thing where you interact with human beings).
Traditional publishing saves you those upfront costs, and hooks you into a sales pipeline that will get your book to readers.
It's all about goals. If you want a small release without the commitment to becoming a professional author, self publish. If you want to eventually transition from the back to back to back to back 10 hour days into a writing career, then it might help to at least query a few agents and just see where your work stands. Might be the next best thing.