One of the key features of Directory Opus is its Explorer Replacement mode.
To clarify exactly what we mean by Explorer Replacement:
Whenever an action is taken that would ordinarily result in a File Explorer window opening, a Directory Opus window (Lister) will open instead.
Explorer Replacement mode is controlled by the options on the Launching Opus / Explorer Replacement Preferences page. Selecting any option other than Don't replace Explorer activates Explorer replacement mode.
On Windows 10 and above, the Win+E is intercepted by a different process to Explorer Replacement, and can be set to open Opus even if replacement mode is disabled, from the Launching Opus / From the Win + E hotkey Preferences page.
When Explorer Replacement mode is enabled, it is still possible to get to File Explorer if desired:
Even when Explorer replacement mode is enabled, there are some cases when Explorer will continue to open:
Opus does not replace the desktop (which is actually implemented by explorer.exe), nor does it replace the standard File Open / Save dialogs in other applications.
Explorer Replacement requires changes to the Windows registry and so is generally unavailable when running a USB exported version of Directory Opus, however there is an option to temporarily make the registry changes needed (which might work, depending on your user permissions, or it might not).