Koreader help

Can someone please explain koreader to me like they would to a 5 year old or a really dumb person? I have an X3 and an X4 with CrossPoint installed and otherwise use my Boox Palma to read from the kindle app. I also have an iPhone and a MacBook. Where do I start? Do I first put it on my MacBook to load all the books and then somehow sync it to my devices? Do I no longer use my SD card to load books? Do I go into koreader from my devices and read from there or does it just kind work in the background? 😵‍💫