You get them via your local library system (if your library participates). They are easy to download. If I recall, you have to do a manual install of the app to Kindle as they block it on the app download page. Overdrive.com for the app and instructions. This along with Kanopy.com for videos is a great reason to have a library card. :)
Yes. 21 days is what my system allows. Can renew if needed if no others are waiting to borrow.