To use iCloud Drive syncing for your vault(s), put them in the iCloud Drive/Obsidian folder. This folder should be created by the app. If you don’t see it, go into the mobile app, enter the vault picker (the vault icon in the bottom-left), and create a new vault. Make sure to enable the “store in iCloud Drive” option. Then, on a Mac where you’re signed into the same Apple ID, open iCloud Drive, and open the Obsidian folder. You can then cut and paste your vault(s) into that folder. Finally, on your iPhone/iPad, go into the vault picker again—your newly-synced vault should show in the list of options.