Cloud requires device access at some point to get a copy (Download).
Think of it this way:
You went to the bookstore and bought 5 novels (the shopping app).
Took them home and put them in the closet (saved in the cloud) .
When you want to read them, you open the closet (login to the cloud), take out a book (use a library/file app), Put it in your backpack (download to device), close the door (logout). NOTE: you have not yet done any reading
The device app to a CAPTIVE $TORE (Amazon, Apple, Google, Kobo....) does this seamlessly for you, but only for Their store (there are hoops to get 3rd party books to pass thru to your device for some stores).
The whole purpose of the 'branded 'App is to DRIVE YOU to their store for purchases. See, Nothing is really 'free'. Your purchases really pay for that FREE app.
My Android tablet (TAB A), has
Kindle for Samsung (they get a cut of the $) for AZW* books,
Moon + (paid version)for EPUB, And
Adobe reader for PDF
I use
Calibre Companion (and local WiFi) to manage the DEVICES Library and Calibre to host the full Library.
For my K4, I use USB to transfer.
[rant]AFAIAC What and when, I read is my private business and I avoid tattle (always phone home) apps by leaving the device in Airplane mode. YMMV[/rant]