Amazon have gone to great lengths to keep amazon books in the amazon "walled garden", meaning it is difficult (not impossible) to transfer amazon books to other readers, especially for those users that are not tech savvy.
The reason is that most ebooks are DRM encrypted, and neither Calibre nor Kobo, can decrypt them natively.
Even the current (and somewhat complex) method of decrypting amazon ebooks will be gone by the end of the month, according to this article...
https://techy-notes.com/remove-drm-from-kindle-ebooks/
But maybe other members that are more up to date can give better advice.