AFAIK, Obsidian on iOS/iPadOS does not expose a method to open and browse Files. There might be a 3rd party plugin for Obsidian to do this, or a setting I’ve never explored, but I’m not looking. OTOH, one can browse DEVONthink files in Files, open one, then “share” it to Obsidian. This, of course, is a slog and I doubt it would spark joy in many users who demand full access to their DEVONthink database documents while using Obsidian on mobile devices.
(There’s always an easy way to access data, but the hard way is what generates conversation.)