most people will probably use a usb cable to iphone<->PC/Laptop<->USB-Stick.
But can it be done iphone<->USB-Stick?
Apple’s devices do NOT come with a file explorer per default. which sucks. So you will have to rely on the quality of 3rd party apps. Sometimes this quality also sucks.
target: backup picturs to usb stick
requirements: The iPhone / iPad must download the free app „iUSB Pro“ from the App Store to run this card reader.Android-Smartphone it needs USB On-The-Go (OTG) functionality.
OTG a standardized specification for USB connections that allows a device to read data from a USB connection without requiring a PC. The device is basically becoming a “USB Host”, which isn’t an ability every gadget has.
for samsung android phones: https://www.amazon.com/UGREEN-Converter-Adapter-Compatible-Samsung/dp/B00N9S9Z0G/