There are two ways to manage your audio tracks in OBS. The first and default is under Settings > Audio. The second is to add your audio device as a source. This second way gives you a lot more freedom and control!
The default way is fairly straight forward with easy drop-downs, but you only get four input options. The second way not only lets you add as many input sources as you want, but you can also selectively remove/add them depending on the scene. If you want a scene to switch to that removes your audio, say a "Be Right Back" scene, this is the way to do it and what I recommend.
First, go ahead and disable all the audio devices in OBS.
Next, right click in your sources and add an Audio Input Capture.
Be sure to name it in a way that matches your naming convention in VoiceMeeter to prevent future headaches. Once you add it you can define your audio device to map here.
Here is what my final sources look like:
Notice that I didn't add my browser audio. For me, I'm playing music using my preferred program. But I did separate it into its own audio cable. That means that anything that I play in my web browser wont be heard on my stream/recording, even though I can hear it myself. This is the kind of magic you can achieve with virtual audio cables. If I wanted to handle times when I did want my browser audio heard I can simply make a new scene and add that as a source only there!
Finally, let's open up our advanced audio properties because we're going to separate the audio tracks.
OBS allows you to manage 6 audio tracks. When you are streaming, you choose only 1 of the 6 tracks to broadcast. But when you are recording, it saves each track into a separate audio stream. For me I keep track 1 reserved for my stream audio, so all four of my inputs are active there. Then track's 2-5 I route my desktop, microphone, discord, and music independently.
Now here's the cool part: if you use OBS to record, now all of this audio is in completely separate, uncontaminated audio streams!
So if you want to go back and edit your videos, maybe take music out, do some further voice processing on your mic or your friends discord audio, or just adjust levels, you can do all of that. Again, I strongly recommend you keep good naming conventions. You can rename the tracks for recording in OBS settings under Output > Audio