This tutorial use for use your iOS device as usb storage for that need to use third party mac application which use for the flash drive called as a pen drive.
Step 1 : Download iMazing for trial version.
Step 2 : Connect your iPhone using mac machine or windows machine.
Step 3 : Then shows all functionality provided by application.
Step 4 : Shows showing numbers of folders then select “Document” direct select it.
Step 5 : Now ready for adding files,document,tar,zip or any files you want add follow below step.
Right click on empty area -> Copy to “Documents” and select your file want to copy.
Step 6 : Now showing processing and after refreshing you can show the file you selected.
That’s It your files are stored into iOS device.
Step 7 : Now discuss for the file download from another machine like windows or mac.
Just right click of your file and select “Copy to Mac” -> Select the location for save.
Hope this may helps lot.