You can create Obsidian vaults wherever you would like. I created my vaults in iCloud Drive on my Mac. As these are just Markdown files, you can use any Markdown capable iOS app (that has access to the Files chooser which includes iCloud Drive) and make any edits or modifications to those files. It doesn't automatically make link suggestions while editing it in an iOS app, but once you are back on your Mac you can easily update those links or just use the suggested unlinked references tool in the side panel. Does that make sense?