I just found this info in the official tech docs, thought I’d share:
On the Raspberry Pi 2B v1.2, 3A+, 3B, Zero 2, and Compute Module 3, 3+ you must first enable USB host boot mode. This is to allow USB mass storage boot, and network boot. Note that network boot is not supported on the Raspberry Pi 3A+.
To enable USB host boot mode, the Raspberry Pi needs to be booted from an SD card with a special option to set the USB host boot mode bit in the one-time programmable (OTP) memory. Once this bit has been set, the SD card is no longer required.
IMPORTANT Any change you make to the OTP is permanent and cannot be undone
NOTE On the Raspberry Pi 3A+, setting the OTP bit to enable USB host boot mode will permanently prevent that Pi from booting in USB device mode.