Actually it's a little more complicated than that.
To extend the functionality of the UEFI firmware environment a developer can write UEFI drivers. These might be baked into the firmware or they may be part of an Option ROM, which may or may not also have a legacy bios-supporting blob of code!
I don't think the UEFI spec determines what the firmware environment must or must not support in terms of hardware devices, however, it does provide an interface for UEFI drivers to use. As such, either this could be built into vendors' firmware, or it could exist in the option rom, depending on how they decide to do it.
For more, see the best practises for driver development presentation, which is mainly focused on telling driver developers to follow the UEFI spec, but hints as to driver placement too. Particularly the page on common OpROM combinations.