I guess the iOS and iPadOS Obsidian will lookup/create/delete vaults in a specific root path in the iCloud.
I don’t know how to change it.
And if you want to use Obsidian on iOS or iPadOS and based on iCloud, you can put the vault file under that specific root path.
And I think you cannot open any file as a vault on iOS/iPadOS Obsidian. But this can be done in the desktop version of Obsidian.