VoiceMeeter Banana, the Advanced Virtual Audio Mixer by V.Burel
vb-audio.com
VoiceMeeter Potato, the Ultimate Virtual Audio Mixer for Windows
vb-audio.com
Those are two different sizes of the same idea. Pick the one that fits both what you're doing now, and what you're likely to do in the future.
When you install it, you also get some virtual speakers and virtual mics. Send audio from an app to one of those virtual speakers, and it appears as an input to VoiceMeeter. Then you can use VM to control it and send it to wherever you want. "A" outputs are physical, "B" outputs are virtual. Send it to a "B" output, and it appears in a virtual mic for OBS or any other app to pick up. Use different virtual devices for different apps, in both directions, and you have separate control and separate content.
If you're doing *some* audio work outside of OBS, then see if you can do *all* of the audio work outside of OBS. Having all of one thing in the same place makes it easier to manage. The mic, for example, can be a physical input to VM, alongside the virtual ones that come from various apps, and no other app uses the mic directly. They're all fed from VM.