If you’ve downloaded an .apkg or .csv file onto your local device (e.g. it’s stored in the Files app), then tapping on the file should allow you to open that file inside AnkiMobile. You won’t be able to import it directly from AnkiMobile - the actions inside the app are for opening the shared deck list, or for downloading from certain types of URLs which probably don’t apply here).
Importing a deck from outside of Anki - #3 by dae
Try opening the .csv file in Files app > Open with AnkiMobile