My Kobo Clara 2 shows up as a USB mass storage device, and I can just drag and drop pretty much any kind of document.
There's also a sqlite database in there that contains, I think, all of the device's settings and other data, including some crypto stuff for the DRM books that I bought in Kobo's store.
It did insist on an account when I first used it, though. This can be worked around by fiddling with the sqlite database, but I just signed up instead.
Download over usb allows you to download the kindle ebook purchased on amazon to your computer. That gives you an offline copy of your ebook. You can then download it to your kindle over usb.
But since you have the file, you can ALSO send this file to another ebook reader. I believe some ebook readers like pocketbook can read .azw files directly. Readers like kobo might need conversion to epub or kepub.
There's also a sqlite database in there that contains, I think, all of the device's settings and other data, including some crypto stuff for the DRM books that I bought in Kobo's store.
It did insist on an account when I first used it, though. This can be worked around by fiddling with the sqlite database, but I just signed up instead.