Not an apple person but: probably a year or more ago, it became no longer possible to buy kindle books via the Kindle app for Android. It's my understanding that you hadn't been able to via the Kindle app for iThings for some time. Google and Apple want you to buy via their own stores. You can navigate to the Amazon site via a browser and should be able to buy kindle books that way. You can also buy via a full computer or via a kindle or Fire device.
You can borrow from Kindle Unlimited through the Kindle app for Android.
None of this should affect your ability to download books you already own, or have borrowed, to the Kindle app and read them on an Android or iThing device, assuming you have downloaded and installed it from the relevant store. (It's a free app on both platforms I believe.)
But things could be different if you're not in the US.
You can borrow from Kindle Unlimited through the Kindle app for Android.
None of this should affect your ability to download books you already own, or have borrowed, to the Kindle app and read them on an Android or iThing device, assuming you have downloaded and installed it from the relevant store. (It's a free app on both platforms I believe.)
But things could be different if you're not in the US.