I have never been comfortable with drivers, am just now learning about UEFI, and am trying to understand how much UEFI creates its own operating system with this example.
When I boot a modern Windows 10 system (which therefore uses UEFI) with a USB drive, I believe the motherboard EFI controller communicates directly with the USB controller until it eventually gets "BOOTX64.EFI" code in RAM, and it then starts the CPU. (I probably should at least write "effectively starts" - Please forgive my oversimplified explanations.)
Then, if that EFI code is specially-designed to use the USB port, the CPU (instead of the EFI controller) does the communication work. USB drivers for this can all sit in the EFI partition, so let's say they are not associated with Windows.
Then, if that code ultimately boots Windows (located on a second partition on the USB drive), Windows' USB drivers will ultimately get used.
So, can we really have these 3 distinct USB modes? To highlight the differences, let me call them "No CPU", "CPU for UEFI", and "CPU for Windows".
Normally, "CPU for UEFI" is ignorable (because "BOOTX64.EFI" is the Windows boot program), so I wonder if maybe this mode is completely killed in favor of "No CPU" (generated by some call from the CPU to the EFI controller).