First I drill down with OneDrive and find the .one file I want to open. I tap it and I get a dialog "How do you want to open this type of file (.one)?"
I already have OneNote (modern UI app) installed so why wouldn't OneDrive give me that as an option?
Then if I go to OneNote there is nowhere an option to actually open a .one file... I can edit the "Personal (Web)" file ok but I want other notebooks not that one...
So how do I do it?