Kindle users can buy all their books from Amazon but also can transfer books from their PC/laptop or any other device using the USB connection that also is required when you charge your Kindle.
Once you connect the cables to your PC and your Kindle Wi-Fi 6”, the device will appear as an USB stick (a storage drive).
Open the drive, copy your .pdf or .mobi book (or whatever extension you are using for your e-book) into the “documents” folder.
You can unplug the cable from your Kindle and try to use it. Press the “Home” button, you will notice that the book you just copied will be the first on the list and also have a “new” mark on its left.
There are many websites that are offering free books for Kindle, you can use the Kindle integrated browser to download them but as the browser is now, it is not very comfortable to browse the web from this device, also writing from the screen keyboard is also not a very easy thing to do.
PDF books are having some issues, their font is very small and unreadable but these files can be converted with a special software called “Calibre” (free), into .mobi files. Kindle can display .mobi e-books with no issues.