OpenedFilesView uses the NtQuerySystemInformation API to enumerate all handles in the system. After filtering non-file handles, it uses a temporary device driver - NirSoftOpenedFilesDriver.sys for reading the information about each handle from the kernel memory. This device driver is automatically unloaded from the system when you exit from OpenedFilesView utility.

The HKLM\SYSTEM\CurrentControlSet\Enum Tree

The HKLM\SYSTEM\CurrentControlSet\Enum registry tree contains information about the devices on the system. The PnP manager creates a subkey for each device, with a name in the form of HKLM\SYSTEM\CurrentControlSet\Enum\enumerator\deviceID. Under each of these keys is a subkey for each device instance present on the system. This subkey, which is known as the device's hardware key (or, device key), has information such as the device description, hardware IDs, compatible IDs, and resource requirements.

The Enum tree is reserved for use by operating system components, and its layout is subject to change. Drivers and user-mode Setup components must use system-supplied functions, such as IoGetDeviceProperty and SetupDiGetDeviceRegistryProperty, to extract information from this tree. Drivers and Setup applications must not access the Enum tree directly. You can view the Enum tree directly using the registry editor when debugging drivers.