Hi Philip
To access the OPF file you need to crack open the ePub. To do this there are various scripts such as https://www.rorohiko.com/wordpress/manuals/fixed-layout-epub-assistant-in-indesign-epubcra wler/
Lynda.com have useful tutorials on producing and editing fixed and flowable text ePubs from InDesign, which as a relative beginner, I think you'll find invaluable:
(I have no connection with Lynda.com!)
Derek