In my experience, this is a Windows issue - not a Nuendo one. I use both MacOs (UA Apollo x8) and Windows workstations (RME UFX+). To summarise & agree with what other have said here: “By definition such programs use the standard I/O of your computer. So you have to route/configure to which signal your"main” outputs listens to".
Yes, and this applies to any DAW or NLE - the issue is that ASIO drivers are only used by dedicated software such as Nuendo but not by Windows and its separate ‘system audio’ variations (macos combines all this together as ‘core audio’). For reference, I’d also check which audio settings you have used for Audition. I can only imagine this has been set to system audio vs. ASIO (& therefore should not be able to instantiate VST plugins?).
So there are two answers for Windows as far as I am aware:
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You need to have an audio interface that supports BOTH ASIO and Windows audio. RME do a very good job of being able to configure that (& as multichannel audio even). Focusrite is another of the few vendors that allow this for Windows although the channel count is more limited. I’m not sure about the details for a Scarlett 6i6, but here is the same information for configuring MME on a 2i2. You could also write to Focusrite of course.
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As another poster mentioned, Windows has audio patching capabilities and various free third party apps can manage this reasonably well (although one can disappear down the rabbit hole). Jack IO for example, but perhaps the best is from VoiceMeeter , would you believe with ‘Banana’ and 'Potato’options (!) for more and more channels of audio routing. In this case you would be routing Nuendo ASIO output to Windows system audio, however, YMMV with latency.
I hope this helps.