i have an older Kindle but the format should be similar for books. There are two ways to get books through Amazon. First go into Amazon website and go to Kindle eReader and then onto Kindle eBooks. Once you are on that page, look up at Search Bar and it will say Kindle Store- just type free kindle book and it will take you to free book page. I then usually sort by AVE CUSTOMER REVIEW at top right drop down tool. Then I choose the ones I want that are free. The next time I turn my Kindle on they are there- nice and neat. OR you can use your kindle to get them. My older kindle uses this process. Up at top it says Books - click- takes you to next page- click on STORE- taking you to store page. Then in search line, type free kindle books. I again sort by average customer review and scroll through finding free ones. Now the scoop on free. I find most are Series or classic books and it is a great way to see a new author. Some are good for a day or a limited time, so get them while free. There are about 25-30 categories if you choose -see more.
If you are using Amazon on your computer to choose the free books, it is better to go back a page to list than to hit continue shopping tab as it will take you back to beginning Kindle page. I rarely pay for a book as the Prime membership allows you to borrow a free book each month and will all the free ones, I have plenty to read. Some of the Kindle Unlimited books are also available as Buy for Free - but be careful there is no small print under it that says (With Kindle Unlimited).
I also use my local library's free eReader system. It will deliver the book to Amazon and you download from Amazon. Mine is available for just two weeks lending period.
Just play with it. If you have questions, just go into Amazon.com customer support and send them a message. They will get back to you. NO live support.
Lots of great reading out there!