MDP2 looks for files in one place - the MDP2 folder, which you can find in the Files app - on my iPad/iPhone - MDP2 folder
It. does not have a "general" file open allowing searching folders and cloud. (This is the older sandbox approach, which Apple is now allowing some exceptions to, but MDP2 has not implemented.)
You can use a sharing service, but opening the file from there (or a web location) is somewhat hit or miss, and then you still need a manual save action to get it filed in the MDP2 folder, and you have to save the filename. So for most reliable performance, manual save it as below.
Use dropbox (or iCloud, etc.) to upload from iPad, then download to the iPhone MDP2 folder, and it should appear in the list under config / open