To use a key file on your device, you can import it when you are asked to select a key file during the unlock process. You can import it from anywhere in iOS Files or from Strongbox’s local documents directory. You can copy the key file into Strongbox using iTunes File Sharing or the iOS Files app. If it is named correctly (see below), Strongbox will also auto detect it when you open your database.
iOS Auto Key File Detection
If the key file is named “.key”, where database name is the filename of your kdbx file, and is in the Strongbox documents directory, Strongbox will detect it and use it automatically to open the corresponding database.
This will also be clearly indicated on the Unlock dialog before you type in your password, so you will know Strongbox has detected it.
So, for example, if you’ve got a database file named my-database.kdbx then you would name your key file my-database.key, and drop it in Strongbox documents.
Notes
Once you’ve dropped it in Strongbox Documents (using iTunes File Sharing, or the Files app, or other method), you can/should remove the key file from iCloud or whatever other cloud storage you might have used.
It is not a requirement that the key file have any specific extension or be any specific form/type of file.
Auto Detection is not required, you can access your key from anywhere within the iOS Files app, that will also work, auto detection is purely for convenience.