I dedrm and store my eBooks with calibre. But that’s just for archiving purposes. I read my eBooks on my iPhone with whatever app I bought them, that can be kindle or Apple Books, whatever is more convenient. In the rare case that I actually load one from my archive, or from the Gutenberg archives, it’s usually ePub, and therefore Apple Books as a reader.

I also own a kindle device, but find that often more cumbersome, so I use that only in places where I don’t want to take my phone, ie beach or other public spaces where I want to move without much clothing.