Calibre doesn't run on the iPad/iPhone operating system - IOS. It runs on Windows, MacOS, and Linux.
There's an IOS version of the Calibre Companion app, but as its name implies its dependent on having access to the calibre library management application. Here's the
MR Calibre Companion forum.
If you look in the
MR Apple Devices forum you'll find info about reader apps.
Tip: make sure you look in the sticky threads at the top of those forums
BR