You go to amazon.com/myk, switch to the Content tab, three dots next to the book, Download & transfer via USB. You then drag-and-drop them into Calibre with this add-on set up: https://github.com/apprenticeharper/DeDRM_tools
There's some initial setup required to get your decryption key (easy if you have their e-reader — just enter the key you'll find in device info, slightly complicated if you don't), but once that's done, the friction for decrypting ebooks is pretty negligible.
You'll have to convert them to epub or PDF to be able to read them on ReMarkable, but that's as easy as right-clicking a book within Calibre and choosing "convert".
Most of the other ebook decrypters are basically slapping some interface on top of this Calibre plugin and hiding it behind a paywall.