There are several ways to connect a USB flash or SD card to the iPhone and listen to music located on it. You can also manage your files with a built-in file manager in our apps Evermusic, Flacbox, Evertag.
The first way is to connect your USB flashcard using standard Lightning to SD Card Camera Reader https://www.apple.com/shop/product/MJYT2AM/A/lightning-to-sd-card-camera-reader
or connect your USB flashcard using Apple Lightning to USB 3 Camera Adapter https://www.apple.com/shop/product/MK0W2AM/A/lightning-to-usb-3-camera-adapter
In this case open the Evermusic, Flacbox, Evertag app and navigate to the "Connect" tab. Scroll down to the "External files" section and tap "Open files..." to activate the "External Files Picker".
On the "External Files Picker" screen move to the root screen and scroll down the "Location" section to find a connected USB device.
Open the device and select the files you want to import to the app.
Imported files located on your USB device and if you want to copy them to the app answer "Copy files" on the confirmation message.
But there is also another option available cause our apps support Apple Certified card readers with "Power Drive" technology and SanDisk iXpand Flash Drives natively.
You can buy them here:
https://www.westerndigital.com/c/usb-flash-drives.connectorList-lightning
https://www.amazon.com/SmartQ-Reader-MicroSD-Transfer-Adapter/dp/B01N26LTEM
https://www.amazon.com/Certified-Lightning-GMYLE-microSD-Adapter/dp/B01IVMG6T2
In this case, insert a card reader or San Disk iXpand Flash to the lightning port and open the application. You will see the "Device connected" message and device information.
Just open the "Connect" screen and scroll to the "Connected accessories" section.
Tap on the connected accessory icon to open it and get the list of files and folders located on the USB device. In some cases other apps may use your drive and set busy state. In this case you will see warning message. Nothing serious - just close all other apps that uses your flash drive or unplug it and plug again.
Once you opened any folder with music you will see all available audio files. Just tap any file to start playback. You can enable equalizer on the player screen or stream audio content to AirPlay (Sonos, Apple TV) or Google Chromecast device. You can use a built-in file manager to COPY, MOVE, RENAME, DELETE your files. You can also add audio files to the music library in Evermusic or Flacbox apps. And of course you can copy audio files from flash drive to you iPhone memory.