[Quoth]

Originally Posted by leebase

The platforms that Apple and Google command - provide something like 5 Billion people the chance to read books from anyone that can make an app.

I read my Kindle books on devices made by Apple and Google.

It is only in the eInk world that a device is all but captive to the store the device maker tailors it to.

Not true for eInk. But close to true for iOS. Less true for Android as most devices have a setting to allow non-Playstore apk apps.

I've had five makes of eInk. All could read DRM free books from any source. Some Amazon books are DRM free. All 80,000+ PD ebooks work on just about any eink ereader and have no DRM.
DRM doesn't actually really work. It's not hard to read ebooks with DRM from Kobo, Google or Amazon outside their ecosystems.
Apple did remove DRM from MP3s. The ebook DRM is doomed.

I'd not bother buying ebooks from Apple Books or Barnes & Noble as it's pointless. They have no DRM content that's not at the same price or cheaper elsewhere on average and it's more work to remove their DRM. Smashwords has no DRM.

It's trivial to convert epub to azw3 for Kindle, or to mobi for very first Kindles or lrf for first Sony, or pdb for earliest phones and PDAs. It's trivial to Download to PC from Amazon and convert mobi, azw or azw3 to epub (all current non-Kindle ereaders) or lrf for first Sony, or pdb for earliest phones and PDAs.

You can't install any app on iPhone or iPad unless it's from Apple Store. You can't run an iOS app on Android or Windows.
You can't convert an Android App to iOS and couldn't install it if you could. It's just about possible, but flaky, to run an Android App on Linux or Windows.
A real reflowable ebook will work on any eink reader, and free viewers on Palm OS, Symbian, Windows, iOS, Android, Linux, BSD etc.
A flxed layout Kindle file will only work maybe on a Fire or Kindle App. It's like a PDF in an Amazon wrapper.
PDFs are not ebooks. They are electronic documents as print replica or for actual publishing on paper. They do need a big tablet or larger eink.

I read my Kindle books on devices made by Apple and Google.

About half of ebooks are read on phones and tablets. About 92% of the English language ones are sold by Amazon. Google doesn't make most Android devices and less than 1% of ebooks are sold by Google, so it's an irrelevant comment. I've sometimes read part of an ebook on a 4.3″ Android phone older than my iPhone. The 32G Apple iPhone was pristine when given to me but only had the factory apps and even several years ago no ereader or mp3 player could be installed. So can't read ebooks on it. The older Android 4 phone still installs players and reader apps from Google's Playstore.