Can I use a Western Digital USB harddrive as a file storage unit for my iPhone 13?

Understand that Hard Disk Drives are notoriously power-hungry - and will likely require more power than your iPhone's Lightning port alone can supply. As such, unless your external HDD has its own source of power, you must either use a powered USB hub and/or use an Apple Lightning to USB3 Camera Adapter - to which you must also connect your iPhone's Power Adapter. The Power Adapter must be rated at least 12W (2.4A @5V) - although a 20-30W Power Adapter is recommended.

• Lightning to USB 3 Camera Adapter

https://store.apple.com/xc/product/MK0W2ZM/A

Don’t be confused by the name of this Adapter - as it is the key to connecting many USB devices to your iPhone or iPad. The adapter incorporates the essential USB Controller/Root Hub that interfaces USB accessories to your device's Lightning port.

iOS/iPadOS supports FAT, FAT32, exFAT (FAT64), APFS or HFS+ with a single partition. Microsoft NTFS and other proprietary formats are not supported.

Yes, you can use the Files app, or any other app you prefer, to do the same thing.

Ref:

• Connect external storage devices to iPhone - Apple Support
• How to use external storage with an iPhone or iPad - digitaltrends

For what you are attempting to achieve in attempting to access a Hard Disk Drive (HDD) from your iPhone, you have no choice other than to invest in Lightning to USB 3 Camera Adapter:

https://store.apple.com/xc/product/MK0W2ZM/A

This Adapter adds the USB Root Hub/Controller that your iPhone requires to connect the HDD.

Connect external storage devices to iPad – Apple Support

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