Guide for Direct Transfer onto USB from iPhone
Plug SD Card Adapter with SD card in it or USB adapter with USB in it, into phone port (make sure it is one compatible with your phone’s connection point – most likely lightning cable). The phone may take a few moments to detect the card or drive.
Go into the ‘Files’ app. Make sure you’re in the main page of the app. The USB will show up on the main page when detected.
Once it’s detected, go into the ‘Photos’ app.
Click ‘Select’ and tap on all the media you want to transfer.
Tap on the share icon at the bottom of the screen (square, with arrow pointing up).
Scroll and select ‘Save to Files’.
Navigate to main page of Files app and select the SD Card or USB drive. You can make a folder in here for your media by pressing the '…' > ‘New Folder’.
Click ‘Save’.