If you have an iPhone, you can put your photos online and download them to any device.
When you say “usb key” do you mean a thumb drive? Any thumb drive can be reformatted to mac or pc by erasing it (even if it is empty). A Mac can use a PC formated thumb drive. But a PC will not read a Mac thumbdrive.
Be aware that reformatting ERASES all the data on the drive.
With the Mac not recognizing the drive, try a different USB port. Most wont work on an unpowered port like the ones on the keyboard. Or the drive has been used to death. Solid State devices have a finite number of rewrites before they goes toes up.
As for your other question regarding plugging your iPhone in to a PC, you have to do some setup on your PC first.
See if this helps: